Allow multi css files and externalize variables

Hi,

I wanted to have a couple of features because the project that I'm working on requires:
* Allow to set multiple entries as atomify-js does.
* Allow to externalize the CSS object variables instead of setting them up in package.json (using a path to a JSON file).

I added a couple of tests and applied some refactors, just to understand better what the code does, I hope you don't mind.

Thanks and regards
We actually landed the updated version of rework-vars yesterday. I think you'll need to do `var(--mainColor)` now.