Add sw-precache

[Hartorn]

https://github.com/goldhand/sw-precache-webpack-plugin
https://github.com/GoogleChromeLabs/sw-precache

See also
https://github.com/facebook/create-react-app/blob/79661396359de40742792d413269a65d01e86661/packages/react-scripts/template/src/serviceWorker.js
https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/public/manifest.json
https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/README.md#making-a-progressive-web-app
https://github.com/GoogleChromeLabs/sw-precache/blob/5699e5d049235ef0f668e8e2aa3bf2646ba3872f/demo/app/js/service-worker-registration.js

https://developers.google.com/web/fundamentals/instant-and-offline/offline-cookbook/