feat: use webpack 3.0 for creating flat module bundles for es2015

[Hotell]

• this will lower library size, because webpack boilerplate would be not needed anymore for simulating import/export
• also consumer bundler will process just one file so bundling will be much quicker
• leverage uglify-es for minifying es2015 code

make no mistake, all of this is already possible with rollup, but since we are using webpack here well ... :) 👍