added `--cache` option for running in “production”

I've had a number of projects where I start using Beefy and when it's ready to ship, I have to figure out a different way of serving it (or deal with the re-bundle delay on each request).

I created this so that I can just change my `npm start` script to be `beefy index.js --debug=false --cache` and throw it up on a server. The first request has the delay for building the bundle, but the subsequent requests serve from the cache.