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
I realized that the latest version of rework-var is expecting into the options object a property called "map" which contains the variables to be resolved. you can see my solution in the last commit.

Into index.js of rework-var, you can find this:

```js
   options = options || {};
    var map = options.map || {};
```