Add option to allow audio/video multistreams

[Nyxiad]

yt-dlp allows multiple audio/video streams to be merged into a single file via the --audio-multistreams and --video-multistreams flags. When passing these flags in, it's important that the only the following formats are available to the user: avi flv mkv mov mp4 webm The user should also be able to choose which streams get merged into the file since it can take up a lot of extra space and download time.

Everything described above comes from the yt-dlp README.

Multiple audio/video streams in one file would allow applications like VLC or Plex to give the user convenient methods for switching languages. Some services, like Crunchyroll, will sometimes put the subtitles in the video itself which then requires separate video streams for each language.