Add support for variables in property names.

This branch add support for variables interpolation in property names.

This is based on #39 and #40 . Hope partents will get merged so that the diff will look better.

It add tests and fix for property interpolation.

The parsing is a bit strange due to css_vendor token.

It also needs more tests, but for now I just wanted to share the current work and will continue on this once parents are merged.

Thanks!