Support subtitles

[brarcher]

For example, mkv supports srt subtitles, but not all video formats support this. MP4 does not, as it only supports soft or hard subtitles.

https://superuser.com/questions/932730/ffmpeg-mkv-to-mp4-conversion-loses-subtitles

Example mkv files with subtitles can be found here:

https://www.matroska.org/downloads/test_w1.html