setting css variables

Hi @mattdesl!

Setting element style as `el.style[property] = value` is not valid for css variables, so `css(el, {'--active': 'red'});` does not work for now.
We have to use `el.style.setProperty(property, value);` in order to enable custom css properties.
I guess that would be nice of dom-css to care about that.
Is it worth PR or that is out of package scope?

Start at the implementation behind the `css(el, properties)` entry point and inspect how each property is assigned to `el.style`. Verify handling for a custom property such as `--active`; done means `css(el, {'--active': 'red'})` applies the CSS variable correctly.

Written by the indexing model from the issue text.