Create a cache-then-network example using window.caches

Thinking a bit more about use cases for `window.caches` that don't involve service workers (see https://github.com/GoogleChrome/samples/pull/103), I remembered that the [cache-then-network](http://jakearchibald.com/2014/offline-cookbook/#cache-then-network) example that @jakearchibald describes could make for a good standalone example.