Add hyphens mixin

Nothing much to say here: it's a mixin for CSS hyphenation!

The hyphens property is not supported in Chrome yet, but it's supported in Firefox, Safari and even IE10. I guess Chrome and Opera will follow soon enough.

Docs here: https://developer.mozilla.org/en-US/docs/Web/CSS/hyphens
Live example: https://developer.mozilla.org/samples/cssref/hyphens.html