Allow selector interpolation in @extend

As the title says, selector interpolation within the `@extend` statement was a feature I find needing every now and then. This is my first time touching a parser, etc. - so let me know if there are any issues. Added a small test case as well.

Should fix #564 as well. I do intend to also take a look at extending within loops as well, if no one else is already fixing this.