Add support for CSS variables

[rsanchez]

Fixes #7.

[xSyki]

Why ts-ignore is here? Can't we do it some other way?

[xSyki]

As i see this won't work if we gave variables in body. Example test that doesn't work:

  it('should read body color variables', function(done) {
    test(
      'body { --test-color: #0000FF; } p { color: var(--test-color); }',
      ['#0000FF'],
      null,
      done,
    );
  });