feat(user/mp3s_to_mkv.sh):Add variant of 'mp3s_to_opus.sh'

- Note: AudioAnchor in the F-Droid repository fails to handle seekback
  of opus files yet handles mkv containers with a single opus audio
  stream okay. Therefore, converting directly into an mkv container is
  the way to go if AudioAnchor is to be used for combining audiobook
  mp3s into a single audio file.