Adds not function for Libsass compatibility

I don’t think ` not ` is safe to use right now due to this missing feature in Libsass https://github.com/sass/libsass/issues/368

I’ve added a `not-()` function that should allow it to return the correct result on both until that issue is closed. I’m not sure if that’s the right syntax necessarily, so I’m happy to make changes based on feedback.
Let's go with `is-not()` then.