Allow Username Override

I have written a patch that will create a new field on the twitter block configuration page that will allow you to override the twitter username that is set up within the widget. This will free up content editors from needing to go to twitter.com to create a new widget every time they want to display a new user's feed. This is accomplished through the use of the data-screen-name attribute within the generated code.

I will attach the patch in a few minutes, I just wanted to get this issue listed so that I had a issue number to associate to the patch I'm creating.