TiddlyWeb plugin to compare tiddler revisions

Usage:
  GET /diff?rev1=<tiddler>&rev2=<tiddler>[&format=<format>]
  POST /diff?rev1=<tiddler>[&format=<format>]

tiddler references are of the form bags/<title>[/<revision>]
(recipes are currently not supported in this context)

supported formats:
* human-readable line-by-line comparison (default; plain text)
* "unified" (plain text)
* "inline" (HTML)
* "horizontal" (side-by-side; HTML)

POST data (JSON representation of a tiddler) can be used instead of a tiddler
reference (rev1 or rev2 URL parameter)
