Add support for custom properties

[Colin Meinke]

As a syntax theme author I would love support for CSS custom properties which allows the following syntax in CSS:

:root {
  --bg-color: rgb( 255, 255, 255 );
}

body {
  background-color: var( --bg-color );
}

It would be great to get the variable class for --bg-color so I can style consistently between CSS, Sass and Less.

[Winston Liu]

Fixed by #99.
Thanks for clearing out the issue list @Alhadis.

[John Gardner]

I'd say #51 can be closed too, because with the exception of nested selectors, there's seriously nothing else to be done.