[MlMt] Small Markdown to HTML preview

[Allie Martin]

Small Bug:
1. disable the message composer/editor preview
2. start a new message
3. edit, adding some markup.
4. toggle on the preview (⌃⌥⌘P)
5. you shouldn't see any of the text you added present. Even if you 
changed your signature during the editing process, this will not show up 
as well.
6. add a new character or simply hit return and all will display in the 
preview as it should.

This doesn't happen when editing a reply.

—
a Curtis m