Add a customized markdown flavor for the passage widget

Summary:
Adds a custom markdown flavor supporting just paragraph/
bold/italics/underline/{{references}}.

Test Plan:
`make test` has a trivial {{ref}} test case
Also used in the passage widget in the next diff.

Reviewers: michelle, joel

Reviewed By: joel

Subscribers: alpert, alex

Differential Revision: http://phabricator.khanacademy.org/D12257