Add alternative, extensible markdown parsing library

Summary:
This will let us
 * only support bold/italic/underline/paragraphs in the passage
   widget, avoiding awkward cases which don't work such as images
   or headings (since these break line numbers and don't make
   sense in a passage)
 * easily add support for passage ref parsing in a non-hacky way
 * eventually replace our smartypants markdown hacks in the renderer

The design goal of simple markdown is simple:
Provide an easily extensible markdown parser library that we can
customize to our needs without modifying the library itself.

Test Plan: `make test`

Reviewers: joel, alex

Reviewed By: joel, alex

Subscribers: alpert, michelle

Differential Revision: http://phabricator.khanacademy.org/D12256