More sophisticated caption generation

Add more "smarts" to caption generation

e.g. New Sentences should usually start a new caption line.

Beware of end-of-caption edge cases (there are many...)

See
https://github.com/classtranscribe/WebAPI/compare/MSToVtt
which was based on convert Angrave's word-to-captions python code. See heuristics here-

https://github.com/classtranscribe/PythonTools/blob/master/transcribe-cli/ms_json_to_caption.py