Add NamerFunc to allow ordinary functions as Namer

[Tiscs]

Added a type NamerFunc, it's an adapter to allow the use of ordinary functions as Namer, it's useful when you want to use a function as a Namer but don't want to define a new type.

// example.go
reflector := grpcreflect.NewReflector(NamerFunc(func() []string {
    return s.svcs
}))
s.mux.Handle(grpcreflect.NewHandlerV1(reflector))
s.mux.Handle(grpcreflect.NewHandlerV1Alpha(reflector))

[CLAassistant]

All committers have signed the CLA.

[jhump]

@Tiscs, I apologize for the long delay before getting you feedback. This seems like a reasonable idea. Please see suggestion to tighten up the Go docs.