Holistic approach to fetch settings

[eshtadc]

Most SW plugins make fetch calls and there is a need to enable authorization to be sent. This seems like something that should be configured rather than defaulted, but it applies to many different plugins. Rather than having a configuration per plugin that has the application developer repeat, it may make sense to have some sort of fetch init configuration specified in the the overall service worker addon. In some ways I could also see this as an odd choice from a purist standpoint because it isn't used by the SW addon directly. The original PR that triggered this is DockYard/ember-service-worker-cache-fallback#9

Creating this issue as a place to discuss pros/cons....