Keep track of repeated tasks by extending @archived() tag

Keep a datestamp of each time that a task is completed,
even if it already exists in the archive. This is useful
for keeping track of recurrent tasks.
Mark previously existing nodes in archive as @archived

When a project is marked as @done in the todo file, the corresponding
item in the archive should be marked as @archived. This change makes
sure that, even if add_path() does not create new nodes, the leaf
node is marked as @archived.

This requires an updated taskpaper library.