[bilibili] Add preliminary support (#2174)

The URL http://www.bilibili.tv/video/av636603/index_2.html does not work yet.

youtube_dl/extractor/__init__.py

@@ -20,6 +20,7 @@ from .auengine import AUEngineIE
 from .bambuser import BambuserIE, BambuserChannelIE
 from .bandcamp import BandcampIE, BandcampAlbumIE
 from .bbccouk import BBCCoUkIE
+from .bilibili import BiliBiliIE
 from .blinkx import BlinkxIE
 from .bliptv import BlipTVIE, BlipTVUserIE
 from .bloomberg import BloombergIE