[twitch] Add support for vods (Closes #4512)
[nerdcubed] Add new extractor

nerdcubed.co.uk describes videos in a single a feed.json file, providing
references to and metadata on >1300 YouTube videos spread across 3 main
channels as well as guest appareances on other channels via a single HTTP
request.

NerdCubedFeedIE transforms this feed into a youtube-dl playlist, preserving
information present in the upstream JSON (allowing zero-cost title/date
matches) and ultimately referencing the embedded YouTube videos.
Credit @willglynn for nerdcubed (#4515)
[xtube] Fix and modernize (Closes #4489)
[npo] Add support for streams (Closes #4276)
[dvtv] Add support for playlists
[dvtv] Simplify
Merge branch 'petrkutalek-dvtv'
[dvtv] Fix thumbnail scheme