Creating a custom Capybara driver

So you're using Capybara to jam out some acceptance specs, awesome. 

But then you're all like, "But I need to write some acceptance specs for a mobile view, but I don't know how to change the user agent in the default Capybara driver to trigger the mobile format"

Don't worry buddy, you just need to create a customized capybara driver...

And then it's as easy as...

UPDATE: 

I forgot to include the origin of this idea. Thanks philly-mac!