Dispatch event after building webpack config

[jthomaschewski]

This implements #1006

Allows the following syntax to freely modify the webpack config without the limitations of the webpack-merge smart merge.

Example:

// webpack.mix.js

Mix.listen('configReady', (webpackConfig) => {

  // remove original rule
  webpackConfig.module.rules = webpackConfig.module.rules.filter(rule => String(rule.test) !== String(/\.s[ac]ss$/))

  // add modified rule
  webpackConfig.module.rules.push({
    test: /\.s[ac]ss$/,
    loaders: [
      {loader: 'style-loader'},
      {loader: 'css-loader', options: {sourceMap: true}},
      {loader: 'sass-loader', options: {sourceMap: true}},
    ],
  })

})

[jameshulse]

It could be handy to document this feature (I can't find it from googling) as I think it is a common use-case to want the final webpack config for posting bug reports.