feature: Add NavigateForResult

I want to have a reusable viewmodel that can emit results back to the calling viewmodel.

If we can get something like a NavigateForResult with a generic type then that would be neat. Interactions are great but I believe they are best for very simple dialogs and simple results. What about a complex object? What if it's prompted as a popup, but as a full page? Yes we can subscribe to events from the ViewModel before passing it through to Navigate.Execute() but that just returns a IRoutableViewModel object and will make the pipeline less neat :D

ChooseContacts = ReactiveCommand.CreateFromObservable(() =>
    HostScreen.Router.NavigateForResult.Execute(new ContactsResultViewModel())
        .Select(contacts => /* transform result */));

We can do it by making a generic IRoutableViewModel<TResult> so the routing commands can listen to it's result.

Start by reading the navigation commands around Navigate.Execute and the IRoutableViewModel abstraction. Compare the proposed NavigateForResult API with the existing routing flow, including popup and full-page navigation. Done means a reusable viewmodel can emit a complex typed result back to its caller through the navigation pipeline.