Subtitle merger into video container (as mkv)

Add support for subtitle merged into video container. See the following resources for ideas regarding implementation:

- https://gordonazmo.wordpress.com/2011/10/11/merge-subtitles-into-a-matroska-container-on-the-command-line/
- `mkvmerge -o output.mkv -S source.mkv new-subtitles.srt`
- `mkvmerge -o myouput.mkv myinput.mkv --language "0:ger" --track-name "0:mytrackname" mynewsubtitles.srt`