Added -throttle:valuesPassingTest:

We've wanted some kind of dynamic throttling twice now, for two different purposes:

 1. When the view associated with a particular view model is offscreen, we want to throttle (but not cancel) the VM's work. Upon reappearance, that work should get full priority again. Using this new method, we can throttle the inputs which kick off the work.
 1. When determining whether to show/hide a loading spinner, we want to throttle its appearance (so it never shows up if the work takes less than, say, 0.2 seconds), but its disappearance should be instant.

It's _really hard_ to write a correct version of this method from scratch, so offering it as an operator will be valuable.