Adds support for revision marks via diffs

Textplay can now read unified diffs as input (see docs), and
process the diff into a screenplay with highlighted revisions.
This update makes extensive changes to almost every part of
textplay to enable this feature.

Processing diffs into FDX files is not supported. This will be
addressed at some point in the future.

Because I couldn't figure out the CSS to render traditional
revision marks, textplay simply highlights revised lines in
yellow. Which, frankly, looks a bit better.