Show images in diffs

Summary:
Before you could see that the URLs changed, but that wasn't very useful.
This extracts all the images within a text diff and plops them at the
end, highlighting the changes.

Test Plan:
* Open difftest.html
* See the images!

Reviewers: alex, joel

Reviewed By: joel

Subscribers: jack, alpert

Differential Revision: http://phabricator.khanacademy.org/D7689