Improve subtitle support

[tomasklaen]

Currently, we only copy subtitles from input to output, and that only when not cutting video, as ffmpeg doesn't provide any subtitle editing filters, so I can't cut them to re-sync to the new output.

Stuff to figure out/fix/handle:

• mov_text subtitles are not supported in mkv containers, need to be converted to srt.
  This can be achieved with a simple -c:s srt.

• Cutting and re-syncing subtitles.
  As far as I know, ffmpeg can't do this on its own? Would appreciate pointers on how to solve this.
  Needs to support at least mov_text, srt, and ass. Are there other common formats not in this list?