Support documentation for css custom properties

[cgero-eth]

Do you plan to support css custom properties?
The pull request for the scss-comment-parser repository could be a starting point :)

[sh-pq]

Our projects at work use CSS variables for their advantages over SASS variables—e.g. they're trivially configurable. LogRocket writes about why they use them here, for example. I would very much like to be able to document these in Sassdoc.