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.
Unfortunately no, that was what I tried first. It will work with Libsass, but `not` seemed to be a reserved work when I tried running it through the Ruby implementation. This was my compromise—it can definitely be something else, but it can’t be `not()`.