implement API for queries via GET (Fixes #350)

Make output path optional when executing queries. If not specified,
the evaluator simply leaves a result temporary collection in place.

Added a TODO on cleaning up these temporary collections.
Well, yes. What makes this work is that `onComplete`'s argument is lazy. That's admittedly not very obvious.

I can't use `Process.eval` because I don't want a `Process[Task, Unit]`, I want `Process[Task, A]` (that happens to yield no values), so I can add it to the end of the scan process without changing the type. It does seem like there should be an easier way, but my mind-reading seems to be off.