Add support for video subtitles

[Chaz6]

Since you cannot listen to the videos, it would be great to be able to read the subtitles!

[hzeller]

Are there examples of videos with subtitles to get somewhere so it can be tested if implemented ?

[Chaz6]

There is a sample with subtitles at https://github.com/Matroska-Org/matroska-test-files/raw/master/test_files/test5.mkv