+feature add option to set end date without start date

[talgalili]

Currently, if I mark a task as done, I want it to get to the done.txt file with the end date.
It is only done if the settings includs a start date.
I would like to not bother with a start date (it clutters the todo.txt), but have the end date appended (I DO care when my tasks have been done)

Thanks!

Tal