Support variables inside @media identifiers

This solution is somewhat hacky, the @media element really deserves a
Node subclass and it's own parsing rules. What's still missing is LESS
variables in grouped media queries like:

    @singleQuery: ~"(max-width: 500px)";
    @media screen, @singleQuery {
        set { padding: 3 3 3 3; }
    }

Fixes #18