Use webpack-config-utils and optimize webpack

[nstrelow]

Closes #29.

• Merged webpack configs into one, using webpack-config-utils. Now everything is in one place.
• Enabled scope hoisting, reduces size in production a bit (20kb). Detailed explanation here.
• Enabled babel-loader caching. Cached in .babel_loader_cache. babel-loader options

I also suggest that we change devtool: 'eval-source-maps' to 'eval', if we do not use the generated js sourcemaps. This would speed up our development build.

An interesting article on optimizations here.
We could still investigate:

• Dynamic Imports: Imports and downloads some components only when needed. IMHO is not useful for us, only for bigger apps.
• CommonsChunkPlugin: Extract node_module packages to vendors. Reduces our main entry point and the file is cached in the browser as long as we don't upgrade our packages. User only needs to download our changed main.js

Webpack 4 should be out soon, this introduces nice default configurations for development and production.

[nstrelow]

Closing in favor of webpack 4 PR: #128