- add metadata for 'series', 'episode', 'season', 'season_id',
'season_number', and 'episode_number'
- integrate series title into the title
- extract subtitles (fallback to `vtt` if the subtitle file type
detection fails as that is the most likely extension)
BrightcoveNewIE.extract_urls(): Handle player data when it is all in
attributes of the <video> tag, and also the id is
data-brightcove-video-id not data-video-id.
Add test to generic extractor, note HEAD gives 404, no checksum computed(?).