Upgrade to Webpack 4

[thymikee]

Merging the branch created by work from #366 and #370.

@czystyl can you make me a favor and test it once more? I've updated webpack to stable v4.1.0

[thymikee]

For now I've verified iOS works, with HMR too.
I get these deprecation warning though:

(...) Tapable.plugin is deprecated. Use new API on `.hooks` instead
(...) The callback argument was splitted into resolveContext and callback.
(...) The type arguments (string) is now a hook argument (Hook). Pass a reference to the hook instead.

[czystyl]

It appeared just after the update

[czystyl]

It looks like everything works except HRM with a Chrome debugger on Android

[thymikee]

As a followup we'll migrate to new plugins API (ref: https://github.com/webpack/webpack/search?utf8=%E2%9C%93&q=Compiler.hooks&type=)