Question: How to extend config and add post-css plugins?

[fredrik-sogaard]

How would I go about extending the config and adding a few simple postcss plugins? I've got it working by modifying createConfig.js directly like so:

{
  loader: require.resolve('postcss-loader'),
  options: {
    ident: 'postcss', // https://webpack.js.org/guides/migrating/#complex-options
    plugins: () => [
      require('postcss-easy-import'),
      require('postcss-flexbugs-fixes'),
      require('precss'),
      autoprefixer({
        browsers: [
          '>1%',
          'last 4 versions',
          'Firefox ESR',
          'not ie < 9', // React doesn't support IE8 anyway
        ],
        flexbox: 'no-2009',
      }),
    ],
  },
}

Is there a simple way of doing this in after.config.js?