Add a couple missing Markdown extensions for e-mail rendering.
Review Request #697 - Created June 9, 2017 and submitted
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/