specify the final target to complete in loop_tasks

now that there are task combiners (#52) there are multiple final targets. it could be convenient to be able to specify which final target you want to see made by a particular loop_tasks call.

Start with the loop_tasks entry point and review the task combiners described in issue #52. Determine how a call could identify one final target when multiple targets exist; done means a loop_tasks call can select and display the requested final target without disrupting existing behavior.

Written by the indexing model from the issue text.