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* Last-modified fields of todos are now updated upon edition.
* Sequence numbers are now properly increased upon edition.
* Add new command ``todo cancel`` to cancel an existing todo without deleting
it.
* Add a new setting ``default_command``.
* Replace ``--all`` and ``--done-only`` with ``--status``, which allows
fine-grained status filtering. Use ``--status ANY`` or ``--status COMPLETED``
to obtain the same results as the previous flags.
* Rename ``--today`` flag to ``--startable``.
* Illegal start dates (eg: start dates that are not before the due date) are
ignored and are removed when saving an edited todo.