Upgrade webpack to 4

[hvelarde]

webpack 4 some interesting enhancements:

• webpack 4 is up to 98% faster
• a new mode property with sensible defaults optimized for either build size (production) optimization, or build time (development) optimization
• CommonsChunkPlugin has been replaced with a set of defaults and easily overridable API called optimization.splitChunks

refs. https://medium.com/webpack/webpack-4-released-today-6cdb994702d4