Make it ready for the Metalsmith CLI

Thanks to @timohofmeijer, this plugins now supports the Metalsmith CLI!

Watch out, this change breaks the API.

Instead of this: ❌

var pseudoelements = require('postcss-pseudoelements');
var nested = require('postcss-nested');

metalsmith.use(postcss([
  pseudoelements(),
  nested()
]));

You now do this: ✔️

metalsmith.use(postcss({
  plugins: {
    'postcss-pseudoelements': {},
    'postcss-nested': {}
  }
}));

Your postcss plugins will automatically be imported by metalsmith-postcss.