More flexible removal of tasks

[dougalstanton]

Some kind of "cleanup" command to remove all the issues that are done, in the case we no longer need to keep them around.

Workflow:

1. Do work, tick off items, add new ones
2. Team status meeting --- reporting items which are ticked off
3. After meeting clean up list by removing completed items
4. Go to 1

Might be best to replace the remove command with something that better suits the needs:

• remove a single task (completed or not)
• remove all completed tasks in (named) branch
• remove all tasks in (named) branch, ie remove branch entirely. (Maybe not; this is just equivalent to rm .git/checklist/$branch so probably not worth the hassle.)
• remove all completed tasks in all branches