#31 Features from animelover1984/youtube-dl

#31 Features from animelover1984/youtube-dl

* Add `--get-comments`
* [youtube] Extract comments
* [billibilli] Added BiliBiliSearchIE, BilibiliChannelIE
* [billibilli] Extract comments
* [billibilli] Better video extraction
* Write playlist data to infojson
* [FFmpegMetadata] Embed infojson inside the video
* [EmbedThumbnail] Try embedding in mp4 using ffprobe and `-disposition`
* [EmbedThumbnail] Treat mka like mkv and mov like mp4
* [EmbedThumbnail] Embed in ogg/opus
* [VideoRemuxer] Conditionally remux video
* [VideoRemuxer] Add `-movflags +faststart` when remuxing from mp4
* [ffmpeg] Print entire stderr in verbose when there is error
* [EmbedSubtitle] Warn when embedding ass in mp4
* [avanto] Use NFLTokenGenerator if possible