[fetch] Observer API for fetch registry (#65)

Continuation of https://www.w3.org/Bugs/Public/show_bug.cgi?id=23878#c20.

> I'd be nice if there was an ~observer API that allowed an application to be notified when new fetch requests are added to the registry - see discussion in [1]. As a related question: when would fetch objects be made visible, when the request is initiated? If so, then that would be sufficient to solve the use case in [1], and if not then we'd have to investigate alternative routes.
> 
> [1] https://github.com/w3c/performance-timeline/issues/13#issuecomment-108097288

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/65