(Bug 5025) Enables markdown parsing for replies-by-email

So. Gmail automatically hardwraps sent messages. We can strive to unwrap it,
with the attendant difficulties (what if the wrapping is in the middle of an
HTML tag? what about multiple line breaks vs single line breaks?), or we can
rely on a library which is meant to handle text like this.

So. Markdown -- HTML still works, along with the various formatting shortcuts
markdown offers.

For a certain subset of users who regularly use their mobile phones, this
should also be easier than switching keyboards to type angled brackets/quotes

We use regular markdown, with a variant of @username to refer to a specific
user.