Sometimes it's useful to cancel a repeater: for example, when you
want to mark a task as definitely DONE.

The attached patch does this by allowing the user to use -1 as the
numeric prefix argument for `org-todo'.

(It does not delete the repeater, instead it sets it to 0 so that
planning information structure is preserved: for example, you can
have a repeater and a 'start warning in X days' cookie.)

Tests and feedback welcome!