Support for server rendering

[insin]

Currently nwb assumes you'll be generating a static HTML file with HtmlWebpackPlugin - this doesn't cut it when server rendering is needed.

Check out the dev/prod webpack asset handling in @mjackson's https://github.com/mjackson/web-starter - the dev middleware portion of this could be added to nwb's express middleware, and we should also generate a stats file for production.