Subtitle format normalization (to subrip)

[robfrawley]

Add support for subtitle normalization by converting all subtitles to simple text-based subrip format. See the following projects/resources for ideas regarding implementation:

• http://karasik.eu.org/software/
• https://github.com/helixarch/subedit
• https://github.com/nwoltman/srt-to-vtt-cl
• https://github.com/ruediger/VobSub2SRT
• https://en.wikibooks.org/wiki/FFMPEG_An_Intermediate_Guide/subtitle_options
• http://www.nikse.dk/SubtitleEdit/ (https://github.com/SubtitleEdit/subtitleedit)
• http://znetcs.pl/ToSrtConverter
• https://pastebin.com/T6DM9xbq
• ffmpeg -i <blah>.ass -codec:s text <blah>.srt

The following provide some additional subtitle resources regarding subtitle specs and other items:

• http://sub.wordnerd.de/linux-subs.html
• http://www.mplayerhq.hu/DOCS/HTML/en/subosd.html
• https://www.matroska.org/technical/specs/subtitles/ssa.html
• https://www.linux.com/news/subtitle-manipulation-tools-linux
• https://www.afterdawn.com/guides/archive/subtitle_formats_explained.cfm