forwardSignalWhileActive as signal operator

[Adlai-Holler]

A category method on signal would increase readability a lot. If you wanna keep the API footprint small I get that, but anyone who uses this a lot will end up implementing it.

-(RACSignal *)rvm_forwardWhileViewModelActive:(RVMViewModel *)viewModel

[jspahrsummers]

IME -forwardSignalWhileActive: is usually applied to an input, rather than a signal chain, which is probably why we haven't needed a category method.

I also find category methods on RACSignal slightly ugly, aesthetically, but that's not a good reason to avoid them. 😝