load config.rb from project, if it exists.

Howdy,

I had a need to further configure the sprockets environment, specifically, adding an additional engine:

` Spar.sprockets.register_engine '.emblem', Emblem::Sprockets::Template `

The question is, where to put this? At first, I added it to my config.ru. That worked fine until it was time to deploy, or test with Capybara. After some experimentation, this is the best I could come up with. I'm not super in love with it, but its simple, and it works. What do you think?