Improve subtitles support

For each language the extractor builds a list with the available formats sorted (like for video formats), then YoutubeDL selects one of them using the '--sub-format' option which now allows giving the format preferences (for example 'ass/srt/best').
For each format the 'url' field can be set so that we only download the contents if needed, or if the contents needs to be processed (like in crunchyroll) the 'data' field can be used.

The reasons for this change are:
* We weren't checking that the format given with '--sub-format' was available, checking it in each extractor would be repetitive.
* It allows to easily support giving a format preference.
* The subtitles were automatically downloaded in the extractor, but I think that if you use for example the '--dump-json' option you want to finish as fast as possible.

Currently only the ted extractor has been updated, but the old system still works.

youtube_dl/YoutubeDL.py

@@ -154,7 +154,7 @@ class YoutubeDL(object):
     allsubtitles:      Downloads all the subtitles of the video
                        (requires writesubtitles or writeautomaticsub)
     listsubtitles:     Lists all available subtitles for the video
-    subtitlesformat:   Subtitle format [srt/sbv/vtt] (default=srt)
+    subtitlesformat:   The format code for subtitles
     subtitleslangs:    List of languages of the subtitles to download
     keepvideo:         Keep the video file after post-processing
     daterange:         A DateRange object, download only if the upload_date is in the range.
@@ -1019,6 +1019,11 @@ class YoutubeDL(object):
                 info_dict['timestamp'])
             info_dict['upload_date'] = upload_date.strftime('%Y%m%d')
 
+        if self.params.get('listsubtitles', False):
+            self.list_subtitles(info_dict['id'], info_dict.get('subtitles'))
+            return
+        info_dict['subtitles'] = self.process_subtitles(info_dict['id'], info_dict.get('subtitles'))
+
         # This extractors handle format selection themselves
         if info_dict['extractor'] in ['Youku']:
             if download:
@@ -1147,6 +1152,53 @@ class YoutubeDL(object):
         info_dict.update(formats_to_download[-1])
         return info_dict
 
+    def process_subtitles(self, video_id, available_subs):
+        """Select the requested subtitles and their format"""
+        if not available_subs:
+            return available_subs
+
+        if self.params.get('allsubtitles', False):
+            requested_langs = available_subs.keys()
+        else:
+            if self.params.get('subtitleslangs', False):
+                requested_langs = self.params.get('subtitleslangs')
+            elif 'en' in available_subs:
+                requested_langs = ['en']
+            else:
+                requested_langs = [list(available_subs.keys())[0]]
+
+        formats_query = self.params.get('subtitlesformat', 'best')
+        formats_preference = formats_query.split('/') if formats_query else []
+        subs = {}
+        for lang in requested_langs:
+            formats = available_subs.get(lang)
+            if formats is None:
+                self.report_warning('%s subtitles not available for %s' % (lang, video_id))
+                continue
+            if isinstance(formats, compat_str):
+                # TODO: convert all IE with subtitles support to the new format
+                # and remove this
+                subs[lang] = {
+                    'ext': formats_preference[0],
+                    'data': formats,
+                }
+                continue
+            for ext in formats_preference:
+                if ext == 'best':
+                    f = formats[-1]
+                    break
+                matches = list(filter(lambda f: f['ext'] == ext, formats))
+                if matches:
+                    f = matches[-1]
+                    break
+            else:
+                f = formats[-1]
+                self.report_warning(
+                    'No subtitle format found matching "%s" for language %s, '
+                    'using %s' % (formats_query, lang, f['ext']))
+            subs[lang] = f
+        return subs
+
     def process_info(self, info_dict):
         """Process a single resolved IE result."""
 
@@ -1253,11 +1305,18 @@ class YoutubeDL(object):
             # subtitles download errors are already managed as troubles in relevant IE
             # that way it will silently go on when used with unsupporting IE
             subtitles = info_dict['subtitles']
-            sub_format = self.params.get('subtitlesformat', 'srt')
-            for sub_lang in subtitles.keys():
-                sub = subtitles[sub_lang]
-                if sub is None:
-                    continue
+            for sub_lang, sub_info in subtitles.items():
+                sub_format = sub_info['ext']
+                if sub_info.get('data') is not None:
+                    sub_data = sub_info['data']
+                else:
+                    try:
+                        uf = self.urlopen(sub_info['url'])
+                        sub_data = uf.read().decode('utf-8')
+                    except (compat_urllib_error.URLError, compat_http_client.HTTPException, socket.error) as err:
+                        self.report_warning('Unable to download subtitle for "%s": %s' %
+                                            (sub_lang, compat_str(err)))
+                        continue
                 try:
                     sub_filename = subtitles_filename(filename, sub_lang, sub_format)
                     if self.params.get('nooverwrites', False) and os.path.exists(encodeFilename(sub_filename)):
@@ -1265,7 +1324,7 @@ class YoutubeDL(object):
                     else:
                         self.to_screen('[info] Writing video subtitles to: ' + sub_filename)
                         with io.open(encodeFilename(sub_filename), 'w', encoding='utf-8') as subfile:
-                            subfile.write(sub)
+                            subfile.write(sub_data)
                 except (OSError, IOError):
                     self.report_error('Cannot write subtitles file ' + sub_filename)
                     return
@@ -1586,6 +1645,18 @@ class YoutubeDL(object):
             ['ID', 'width', 'height', 'URL'],
             [[t['id'], t.get('width', 'unknown'), t.get('height', 'unknown'), t['url']] for t in thumbnails]))
 
+    def list_subtitles(self, video_id, subtitles):
+        if not subtitles:
+            self.to_screen('%s has no subtitles' % video_id)
+            return
+        header_line = 'Language    formats'
+        sub_lines = [
+            '%-12s%s' % (lang, ', '.join(f['ext'] for f in reversed(formats)))
+            for lang, formats in subtitles.items()]
+        self.to_screen(
+            'Available subtitles for %s:\n%s\n%s' %
+            (video_id, header_line, '\n'.join(sub_lines)))
+
     def urlopen(self, req):
         """ Start an HTTP download """