Use shlex.split for --pp-params and update related docs.

youtube_dl/options.py

@@ -689,8 +689,8 @@ def parseOpts(overrideArguments=None):
         help='Encode the video to another format if necessary (currently supported: mp4|flv|ogg|webm|mkv|xvid)')
     postproc.add_option(
         '--pp-params',
-        dest='pp_params', default=None,
-        help='Extra parameters for video post-processor. The params will be splited on spaces.')
+        dest='pp_params', default=None, metavar='ARGS',
+        help='Extra parameters for video post-processor.')
     postproc.add_option(
         '-k', '--keep-video',
         action='store_true', dest='keepvideo', default=False,