Add a way to retrieve the warnings from Elm Compiler

[halfzebra]

As of today warnings are printed out with a console.log call

This is not ideal for situations, where users might want to do something with the warnings, e.g. pass them down to Webpack or apply some additional formatting.

I don't really have a good idea on how exactly the API for this could look, but it would improve elm-community/elm-webpack-loader

I'm pretty open to the idea of working on a PR for this, I just need some guidance in terms of API design(if the suggestion above makes sense) 🙂

[halfzebra]

Closing this because of elm-community/elm-webpack-loader#141 (comment)