Add batch download support for youtube channel and playlist

[jmbannon]

For massive downloads of videos from channels or playlists, we should support batch downloading out of the box.

For playlists, batch by index. For channels, batch by date interval (day, week, month).
For each batch, process all the files end-to-end. This can be done by invoking download multiple times using different yt-dlp args for rejecting videos that are not in the range.