Add ellipsis support when calling a mixin

This adds support for expanding arguments that were collapsed using  `.mixin(@foo; @rest...) { }` by being able to write `.mixin2(@rest...);`

Also, thanks for adding plugin support! If you're interested, I've used it for:

1. [dead-simple CSS Code Coverage](https://github.com/philschatz/css-coverage.js) for Mocha/Jasmine+BlanketJS and http://coveralls.io (and [Demo](http://philschatz.github.io/css-coverage.js/test/mocha-demo))
2. [CSS Polyfills library](https://github.com/philschatz/css-polyfills.js) that implements several CSS3 Module Drafts from W3C (and [Demos](http://philschatz.github.io/css-polyfills.js/))