implement sentence breaking

Implement option to break paragraphs at sentences, as described in the
README. Per the same, this tends to produce better diffs, by breaking
the text into lines at non-arbitrary places such that there is a better
likelihood of matching unchanged lines.

This is the proper version, which recognizes several punctuation marks
for ending sentences, also when followed by additional punctuation (e.g.
quotes, parentheses) as well as spaces, and can ignore abbreviations.
(The proof of concept implementation - not committed - only and naïvely
broke at period followed by space.)