Snugug clearfix

Homygod Clearfix!

Update for #1151, will resolve #974 and #386.

The overflow clearfix method, while super small, in most cases isn't desired as it does more than just fix floating issues. A clearfix should, by default, only fix floating issues. If someone wants to use the overflow method, that should be called explicitly.

As for all of the extends, they're there to reduce overall output. I'm open to debating whether or not it should be enabled by default, but IMO most uses of clearfix don't happen from behind media queries and as such the extends should be fine.