Feature/more caption formats

Allowed the importing of captions with the .vtt extension. The format is very similar to srt and by changing a few lines it added basic ability to import them. When there is more options for the subtitles (size, font...) this will be changed as webvtt supports inline styling for the captions. As for now though this works. (If there is already the ability to change caption characteristics let me know, I didn't see any options to change them)