Progressive Web App support

[insin]

Originally from #166:

Preact's tiny size makes it really compelling for Progressive Web App usage, so how about investigating sw-precache-webpack-plugin with a view to adding nwb new preact-pwa <name>? react-hn could likely use whatever new config we have to provide/support, too.

Still need to investigate how we can support building PWAs and enable that for an app with a flag or config setting.