allow multiple spaces between single-line key:value pairs
First steps to revision-marks support.

- Adds/Changes CSS to display line on right side for
  revised elements.

- Adds prelim markup for html
Phrasing update
Bugfix for detecting scene-headings

If the first word on a line started with "Ext" or any other standard
scene-heading prefix - it would incorrectly tag that paragraph as a
scene-heading.
Added textplay-diff, tweaks to revision CSS

* textplay-diff accepts diffs and generates textplay markup
  for processing by `textplay`.
* Minor revisions to CSS
Major improvements to dual-dialogue rendering
Removed prelim support for revision-markers
Removed textplay-diff