The Markdown e-mail rendering code wasn't correctly rendering lists or
strings with double-underscores separating words, due to missing a
couple of extensions. This adds those missing extensions, bringing some
consistency.

Testing Done:
Tested the e-mail previews before and after this change. Saw that the
Markdown had rendered incorrectly before in these cases, but rendered
correctly after.

Reviewed at https://reviews.reviewboard.org/r/8992/