Add an option for *not* adding the CLOSED property on DONE

[wilderjds]

• I have searched for existing issues that may be the same as or related to mine.

It would be useful if you could add an option to prevent creation of the CLOSED property once a task has been marked as done; the reason for this request is that it causes unnecessary conflicts if the task has been marked as DONE also on another machine (perhaps with no timestamp or with a different timestamp)