@mention allow for custom regex to identify usernames.

* This allows for easier customization of the mention patter regex
This is a simple and clever solution. :+1: 

Are the tests passing for you? The [Hash#new](http://ruby-doc.org/core-2.1.3/Hash.html#method-c-new) docs suggest that you're responsible for storing the value. I haven't tried running it yet though.

> It is the block’s responsibility to store the value in the hash if required.