Add compiling method that groups styles by Selector

Currently we get our compiled CSS to group by `property: value`.

```
.c-component {
  property-1: A;
  property-2: B;
}
```

We can try using a postCSS plugin that will do this for us (hopefully).

First run:
https://github.com/Cheppers/postcss-expand-selectors

Then:
https://www.npmjs.com/package/postcss-combine-duplicated-selectors