Add support for CSS Selector Level 4 variables

[renestalder]

While not stabilized by the specification, it grows in popularity due to PostCSS and it's plugin possibilities. Some browsers already implemented them and I use them for about 6 months in my projects with help of PostCSS and PostCSS Next, replacing Sass for a future friendly alternative.
https://drafts.csswg.org/css-variables/

It might be worth a look for an implementation in kss.