Add support for posting with Markdown-formatted commit messages.
This adds a new option --markdown option to rbt post which prevents

escaping of content within the commit message for the description. This

prevents the need to unescape the description before publishing if the

commit message is considered Markdown-safe, or if it's otherwise

Markdown-formatted.
There's also a MARKDOWN option for .reviewboardrc that can be set to

enable this by default.
This only works for Review Board 2.0+. For now, it may cause an error

when using it with older versions. We'll likely want to introduce a

capability and make use of it.

Posted this change, and didn't touch the Description field.