simple-markdown: add headings

Summary:
Adds

    # heading

and

    heading
    =======

style headings to simple-markdown

Note: regexes are mostly copied (with slight modifications, mostly for
the double newline requirement at the end of block elements) from
marked.js.

Test Plan:
`make test`

probably not as comprehensive as I'd like but shipping beats perfection

Reviewers: emily, kyle

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D12488