Add Mac / iOS hooks to app delegates for things like push notifications

[richardeakin]

Need to think of a flexible way for folks to make use of things like push notificiations, and other things that require configuration with the app delegate. I was thinking a possible direction would be a cocoa-specific applicationDidLaunch() signal that also provides the (NSDictionary *)launchOptions.

I'm not sure exactly what else is needed to enable these features, or what else we could / should expose, so input from iOS / Mac users out there is most welcome.

Carried over from #959.

[pizthewiz]

Getting a signal on application(_:didFinishLaunchingWithOptions:) I think would be a good first step, as that it typically where one will install/configure services like push notifications, analytics/event logging, custom crash reporters and the like.