@mention provides a context variable for overriding username pattern

* This allows for easier customization of the usernam mention pattern regex
Merge pull request #157 from brittballard/mentions-starting-with-underscore

@mention allow for custom regex to identify usernames.
Revert "Search for text nodes on DocumentFragments without root tags"
Merge pull request #158 from jch/revert-146-search_text_nodes

Revert "Search for text nodes on DocumentFragments without root tags"