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

[jeffposnick]

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 example that @jakearchibald describes could make for a good standalone example.

[chawapbebe]

@ithinkihaveacat