Changing RoutingState.NavigateBack Result type to IRoutableViewModel.

I think RoutingState.NavigateBack should be 
```cs
ReactiveCommand<Unit, IRoutableViewModel>
```
instead
```cs
ReactiveCommand<Unit, Unit>
```
This is more in line with the rest of the commands in this class and is more useful when you need to know the past screen (since it is removed from the stack during this command).
There is also an alternative here to return the current vm. 