Allow configuration of which method "clearfix" uses

The designers I know and have worked with all know what a clearfix is, and I think many assume that a project will contain a `clearfix` mixin, whether it comes from Compass or not. 

I do not encourage use of the `overflow: hidden` clearfix as I dislike its side-effects.

That is to say, I see real value in changing the default clearfix implementation in our project, as opposed to asking everyone to remember to use a different clearfix mixin.

This should enable that.

I can/will contribute documentation as well if this PR is likely to be accepted…?