Add script to auto-generate colors.js module of ustyle colors

[domtronn]

I thought it might be nice to include all of the ustyle colour variables in ustyle-react so that you would be able to do things like the following

import { cRed } from 'ustyle-react/dist/colors'

export default () => (
  <Icon fill={cRed} />
)

In order to keep these aligned with ustyle itself, I thought it would be best to generate this file rather than having to manually maintain the map (even if the colours don't change very often). So I wrote a script which will pull out all the colour variables from the _colors.scss and convert them into a ES6 module.