Make hashtags at the end of bolded text render properly

Summary: Fixes T10096. The ** was being parsed as part of the hashtag, so **#asdf** interpreted #asdf** as a hashtag.

Test Plan: Unit test; bolded stuff with hashy contents.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10096

Differential Revision: https://secure.phabricator.com/D15006