(bug 56684) Use HTML formatters in diffs.

Using HTML formatters in diffs allows us to nicely visualize
complex DataValues like Quantities or Geo-Coordinates.

This introduces "HTML for diffs" as a new output format.

Change-Id: Ie852fce9fbb19c58beb21b88e9af16dcb84ff0d5
Merge "(bug 56684) Use HTML formatters in diffs."