index.js

const cloneDeep = require('lodash.clonedeep')
const path = require('path')

const ruleChildren = (loader) => loader.use || loader.oneOf || Array.isArray(loader.loader) && loader.loader || []

const findIndexAndRules = (rulesSource, ruleMatcher) => {
    let result = undefined
    const rules = Array.isArray(rulesSource) ? rulesSource : ruleChildren(rulesSource)
    rules.some((rule, index) => result = ruleMatcher(rule) ? {index, rules} : findIndexAndRules(ruleChildren(rule), ruleMatcher))
    return result
}

const findRule = (rulesSource, ruleMatcher) => {
    const {index, rules} = findIndexAndRules(rulesSource, ruleMatcher)
    return rules[index]
}

const cssRuleMatcher = (rule) => rule.test && String(rule.test) === String(/\.css$/)

const createLoaderMatcher = (loader) => (rule) => rule.loader && rule.loader.indexOf(`${path.sep}${loader}${path.sep}`) !== -1
const cssLoaderMatcher = createLoaderMatcher('css-loader')
const postcssLoaderMatcher = createLoaderMatcher('postcss-loader')
const fileLoaderMatcher = createLoaderMatcher('file-loader')

const addAfterRule = (rulesSource, ruleMatcher, value) => {
    const {index, rules} = findIndexAndRules(rulesSource, ruleMatcher)
    rules.splice(index + 1, 0, value)
}

const addBeforeRule = (rulesSource, ruleMatcher, value) => {
    const {index, rules} = findIndexAndRules(rulesSource, ruleMatcher)
    rules.splice(index, 0, value)
}

module.exports = function (config, env, plugins) {
  // Add CSSnext plugins
  const postcssLoader = findRule(config.module.rules, postcssLoaderMatcher)
  const oldPostcssPlugins = postcssLoader.options.plugins()
  const autoprefixerIndex = oldPostcssPlugins.findIndex(x => x.postcssPlugin === 'autoprefixer')
  let autoprefixerOptions = {}
  if (autoprefixerIndex !== -1) {
    autoprefixerOptions = oldPostcssPlugins[autoprefixerIndex].options
    oldPostcssPlugins.splice(autoprefixerIndex, 1)
  }
  const postcssPlugins = [
    require('postcss-import'),
    require('postcss-cssnext')(autoprefixerOptions)
  ].concat(oldPostcssPlugins).concat(plugins)
  const newPluginsFun = function () {
    return postcssPlugins
  }.bind(postcssLoader.options)
  postcssLoader.options.plugins = newPluginsFun

  // Add CSS modules
  const cssRule = findRule(config.module.rules, cssRuleMatcher)
  const cssModulesRule = cloneDeep(cssRule)

  cssRule.exclude = /\.module\.css$/

  const cssModulesRuleCssLoader = findRule(cssModulesRule, cssLoaderMatcher)
  cssModulesRuleCssLoader.options = Object.assign({ modules: true, localIdentName: '[path][name]__[local]' }, cssModulesRuleCssLoader.options)
  addBeforeRule(config.module.rules, fileLoaderMatcher, cssModulesRule)

  return config
}