simple-markdown: add image+refimage support

Summary:
    This adds support for inline ![](images) and
    [ref][images]

    [images]: <url>

This change also renamed LINK_HREF to LINK_HREF_AND_TITLE
to be more accurate about what it is capturing

This change also fixes titles on regular inline links, and
adds test cases verifying that

Test Plan: Make both types of images in the simple-markdown-tester

Reviewers: emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D12709