Adds a helper function that extends padrino's link_to to check if the cu...

...rrent page URL matches the URL in the navigation and add the twitter active class

This was such an easy fix that has vexed me. I knew there had to be some way to do this but I hadn't really understand the right way within the middleman process to figure where to do this myself.
I got lucky and found https://github.com/middleman/middleman/issues/303#issuecomment-4362124