allow vars argument

Basically allows `rework.vars()` to have an optional argument `map`. This way you can define your variables in a JSON file instead of in your CSS file. Overwrites the map with additional variables, but I don't think that would be an issue. Tried to change as little of the code as possible.

If you accept:
- Consider removing var support in CSS. Having a single JSON file makes more sense (at least to me).
- Add CLI option `--vars variables.json` (optional if you keep inline var support)