[BeatportPro] Add new extractor

This extractor is for Beatport's 2-minute, low-quality track previews
only.  To obtain an entire track, you obviously have to purchase and
download it normally through the Beatport store!

Possible future improvements:
- Playlists for albums or other track-list pages
- User login to play from My Beatport, Hold Bin, or Cart

youtube_dl/extractor/__init__.py

@@ -37,6 +37,7 @@ from .bandcamp import BandcampIE, BandcampAlbumIE
 from .bbccouk import BBCCoUkIE
 from .beeg import BeegIE
 from .behindkink import BehindKinkIE
+from .beatportpro import BeatportProIE
 from .bet import BetIE
 from .bild import BildIE
 from .bilibili import BiliBiliIE