Opinionated minimal webpack 3 config for JS/CSS stuff

• Enforce the use of the Webpack 2+ API through Typescript definitions. I.e. the use of the deprecated Webpack 1 API will throw errors.
• Automatic prettier-standard formatting for JSON, JS/JSX, TS/TSX, CSS/SASS/LESS
• Basic PostCSS support
• Basic production/dev configuration split:

  yarn run dev   # launch webpack-dev-server
  yarn run build # production build with minification, etc.

• Webpack HotModuleReplacement enabled for dev
• Basic Babel env setup for modern browsers (>1% global usage and last 2 versions)
• Basic Babel performance tweaks: directoryCache and babel-runtime
• No other dependencies beyond the "basics".