Stay on the old route while loading the next

[Multiply]

Moved the discussion here (from #22), as per @tmeasday's request.

IRP should allow staying on the old route, while loading the next. It's how I originally imagined IRP to behave, so this should also turn into the default behaviour.

Currently it will show an empty page with the progress bar, unless you've defined a loadingTemplate. If you have, it'll show that and a progress bar.

[fvpDev]

Is this feature the cause of the following issue?:
ManuelDeLeon/viewmodel#151