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.
`Process` could use a little helper, something like:

```scala
def cleanupWith[B](f: Task[B]): Process[F, A] = this.onComplete(Process.eval(f).drain)
```