Introduce an enableOffline configuration flag

[aboodman]

Introduce an enableOffline configuration flag that controls whether offline features work. Defaults to false in dev mode, true in production.

Addresses #143

[shadowwalker]

Thanks for the pull request.
This is something I was trying to avoid initially as it adds complexity.
For offline, you can simply verify it works using yarn start instead of yarn dev. Once offline is working, it's working.
During the development mode, most caching rules are disabled to avoid noise when developing other service worker features, such as web push. You will keep online during development mode anyway since hot reloading needs that to work.
Please feel free to argue a scenario where full offline is a must during development.

[ImBIOS]

This PR is stale, right? @shadowwalker
Do you mind adding this to the backlog or something? I recommend you close this PR if it's not a backlog or something to implement.

[ImBIOS]

@aboodman It seems this repo is dead and the maintainer cannot be contacted for so long now. I've made an immediate response by resurrecting this repo ImBIOS/next-pwa.