[Feature] Identify and fail files with embedded WebVTT subtitles.

**Is your feature request related to a problem? Please describe.**
Plex runs ffmpeg, which does not properly support mkv files with embedded WebVTT subtitles, causing plex playback to immediately fail unless subs are disabled.

**Describe the solution you'd like**
Expose an option to check for WebVTT subs and fail the files that contain them.

**Describe alternatives you've considered**
I don't know of another workaround. When I've encountered this in the past I had to convert the embedded subs to an external crt and strip the WebVTT one from the file.

**Additional context**
I'm not sure how often people run into this, so I can see it being a low priority addition.