Custom props & media config in webpack

webpack.config.js

const customMedia = require('./postcss/customMedia');
const customProperties = require('./postcss/customProperties');

module.exports = {
  ...
  plugins: [
    postcss: () => [
      postcss: () => [
        cssnext({ features: { customMedia, customProperties } }),
      ],
    ]
  ]
  ...
}