@mention allow for custom regex to identify usernames.

* This allows for easier customization of the mention patter regex
This test seems too tied to the implementation to me. It's testing the definition of `Hash.new` rather than actual behavior. I think we can do without it. I think you can roll this test into the one below, and assert that filtering with the default regex leads to a `pattern_count` of 1, and filtering again with a custom regex gives a `pattern_count` of 2.