Faintly outline lines with changes in text diffs.

Summary:
{F14426}

There's some somewhat hairy logic for splitting up a diff into multiple
lines, I tried to ease the pain by adding tests, but it's still not
especially pleasant.

Currently the perseus tests are not working ( :( ) so if you want to run
the tests for just the diff splitter you'll have to comment out the
other ones in test/test.js

Test Plan:
* Load up difftest.html
* Unfortunately perseus's unit test runner is not functional at the
  moment due to require stuff, but I ran the tests in the browser and
  they worked.

Reviewers: alex

Reviewed By: alex

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7390