support an `after` method on generators

[knomedia]

Calling an after method on generators as the last action in the runner would allow for builders of origins to run their own cleanup code as well as prompt users for needed action.

The originate-origin origin attempts to do so, and makes users feel as though this will be the case (see here for where that is happening).

I'm down to do the work to make this happen, if it's something that you're interested in having loom support.