[BACKEND] Always filter out live, post_live, and upcoming videos on download (#742)
Closes Issue: #729
Add a new field to the match_filter plugin: download_match_filters. This set of filters will be applied at the download stage (as opposed to filters which get applied at the metadata stage). By default, always set "!is_live & !is_upcoming & !post_live" as a download filter. This prevents ytdl-sub from hanging when trying to grab live and upcoming videos.
Docs on match-filters: https://ytdl-sub.readthedocs.io/en/latest/config.html#match-filters