Media cache on by default #12

A proxying instance with no cache re-fetches every image from wixmp on every view. Set cache.enabled: true in the built-in defaults in app/config.go and in config.example.json, with a sane lifetime and max-size. Keep memcache off. Document the change in SETUP.md.

Verify: fresh start with no config writes to the cache directory.

Roadmap item 1.5, size S. See ROADMAP.md.