Provide flag to enable webpack 'debug' or 'production' mode

[mfellner]

Webpack offers a debug (for source-maps etc.) and a production (for optimisation) mode.

It would be great to enable either mode in a Lektor build.

Another configuration option that is frequently used in webpack projects is setting the NODE_ENV variable. Webpack can inject its value with the DefinePlugin into code in order to further optimise it. It would make sense to do this when running webpack in the 'production' mode.