Add webpack.debug config to create a more debuggable production build

[insin]

Where a "production build" is a build which has development-only and other dead code removed.

Using this should:

• Trigger use of NamedModulesPlugin
• Configure UglifyJSPlugin with {mangle: false} (don't disable compress as it's what removes dead code)

Docs should encourage use of this via a --webpack.debug argument.

Always emit a warning when this is enabled via a user config file, so it's harder for someone to forget to remove it after adding it temporarily.

Inspired by playing about with nwb web build for webpack/webpack.js.org#1331 to figure out how tree shaking actually works - it was fiddly to pass all the webpack.uglify options and there was no way to conditionally enable NamedModulesPlugin while keeping other production build features)