CSS Challenges

Summary:
Includes CSS matching code, wildcard/callback code and a test suite.
This is just the first draft, will improve the available built-in functions
in response to Pamela's needs.

Test Plan:
Run test suite
Try in browser

Reviewers: pamela, john

Reviewed By: john

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D13378