Checklist

• I'm reporting a feature request
• I've verified that I'm running yt-dlp version 2021.12.27. (update instructions)
• I've searched the bugtracker for similar issues including closed ones. DO NOT post duplicates
• I've read the guidelines for opening an issue

proposal

--sub-format currently only selects 1 format.
It would be nice to allow multiple formats.

syntax

examples:
--sub-format ttml,vtt,xml downloads these 3 formats
--sub-format all downloads all formats
--sub-format ass>srt>best downloads 1 format in this preference (already possible)
--sub-format ttml,vtt>srt,xml>smi,best combination of list and preference

use cases

Subtitles are tiny (compared to the video) and can have a multitude of purposes (e.g. not every player or software can handle all formats equally well and there are many possible issues with converting from one format to another), so it can be very helpful to have several versions ready, e.g.:
      ✅ for human readability
      ✅ for compatibility with different apps and hardware players
      ✅ for embedding in container files (not all container accept all formats)