Mode to generate static HTML files

[srid]

Add say --generate to run Ema in the "generate" (not dev server) mode, that ends up generating the .html files once and quits.

• Allow user to specify a model -> [route] function that gives us the universe of routes (we need this to know which routes to generate)
• Make sure to disable any dynamic part of the Ema app
  • No websocket shim
  • Exceptions during route render (see Display Haskell exceptions in browser #7) should be handled and displayed on the CLI, rather than left in HTML.