Allows @extend to use mixins

This patch simply lets mixins be extended as if they were css blocks.



    foo(){
       div.bar {
           background-color: red
       }
    }

    .myclass {
       @extend foo
    }

    .anotherclass {
       @extend foo
    } 


Output will be:

    .myclass div,
    .anotherclass div {
      background-color: red;
    }

If instead, you used the mixins normally, you wouldn't get the optimization of the combined selector. In the future, it'd be cool if we can do `@extend border-radius(4px)` and anywhere else that is used, will be combined using a comma selector. This is one step toward that direction.