Allow to inject a custom config

Created by: caub

The idea is to allow customizing the config, example:

const webpack = require('webpack');
const MomentLocalesPlugin = require('moment-locales-webpack-plugin');
const build = require('react-scripts/scripts/build');
const config = require('react-scripts/config/webpack.config.js')('production');

config.plugins.unshift(
  new MomentLocalesPlugin({
    localesToKeep: ['es', 'fr'],
  })
);

build([], webpack(config));