Add an example for responsive, horizontal overflow navigation

Hey Brad,

I noticed that the link for a "Horizontal Overflow" navigation pattern was inactive, so, I activated it! I found a pretty neat example on Codepen that does not require any JS, check it out: http://codepen.io/jordanmoore/pen/pnlAi

If that is what you meant by "Horizontal Overflow", feel free to merge this in.

Thanks! :+1: 
Chris