simple-markdown: make mailto: links comply with standard markdown

Summary:
[Standard markdown][1] preserves mailto: or MAILTO:
prefixes in the output of mailto autolinks, if the
author included the mailto: in the autolink

[1]: http://standardmarkdown.com/

Test Plan:
`make test`
ran the standard markdown tests and saw the following one pass:

input:

    <MAILTO:FOO@BAR.BAZ>

output:

    <p><a href="MAILTO:FOO@BAR.BAZ">MAILTO:FOO@BAR.BAZ</a></p>

Reviewers: emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D12857