Add vendor support for Wistia video

While Wistia has it's own [responsive plugin](http://wistia.github.com/demobin/video-foam/), FitVids is really well known, and it would be cool for customers to be able to use both options (especially if they already have a FitVids implementation in place).

Will also add documentation on our side for the `customSelector`, but out-of-the-box support would be neat too :wink:

Added a test for review. Rock on!