Extending bootstrap with Themes

### Background

I had a brief chat with @mdo earlier this week about adding hooks to Bootstrap components with the goal of 1) standardizing a simple way to extend bootstrap and 2) optimizing the compiled output.

Here's an idea that might solve the first half of that.

A couple of caveats:

- This method requires the addition of a hook mixin at the end of every themeable mixin
- To avoid less compilation errors, this method requires all mixins to exist in a theme file, even if there are no custom properties defined.

**As far as optimizing the output:** I realize that this is probably crazy talk, but it would be interesting to have the means in less to actually *not* cascade a specific definition, but supersede it before compiling. Any ideas? Let's discuss.