This PR adds a Babel-like PostCSS plugin config format.

new EmberApp(defaults, {
  cssModules: {
    plugins: [
      require('postcss-calc'), // without any further config
      [require('postcss-preset-env'), { stage: 4 }] // with extra config
    ]
  }
});

This format, compared to the standard config format, allows ember-css-modules plugins to patch the config for PostCSS plugins. This is very useful for plugins like postcss-mixins, so that multiple ember-css-modules plugins can add mixin definitions to the same postcss-mixins instance. This would fix: postcss/postcss-mixins#104

The alternative for me would be to have a "main" ember-css-modules plugin that adds postcss-mixins and has something like a mini plugin registry itself, so that other ember-css-modules plugins can hook in there. This seems unnecessarily complex to me though.

Do you think this feature is a good idea? What would you do differently?