Support for staging builds

Created by: ehtb

Hi, I thought about adding support for staging builds. I am not sure if this requires ejecting though, or could be a feature of CRA.

Our use case is that we create staging builds to deploy to our staging servers. This allows us to debug the app in a production like environment, but with the benefit of non-minimised code including proper sourcemap support and still includes the development warnings and notifications, as well as devtools support.

It can look very similar to webpack.config.prod.js, but with changes to L52-L54 and removal of webpack.optimize.UglifyJsPlugin.