Add support for ELM_DEBUG environment variable

[JohanWinther]

Problem

When developing the UI it can be a bit cumbersome to manually edit webpack.dev.js to enable Elm's debugger and also there's a risk of committing this change.

Solution

The dev server reads from the environment variable ELM_DEBUG to set enable Elm's debugger.

Caveats/Notes

I think I recall seeing some documentation that the Elm executable itself supporting this environment variable, but I can't find the documentation for this right now. The env variable is not supported natively by the elm-webpack-loader though (I checked).

[hojberg]

Sweet!