do: no safeguard to do twice

[ginatrapani]

If by accident you repeat @todo.sh do@ twice on the same task, this should be ignored but instead we have:

$ t add test
TODO: 'test' added on line 114.
$ t do 114
114: x 2009-03-21 test
TODO: 114 marked as done.
$ t do 114
114: **x 2009-03-21 x 2009-03-21** test
TODO: 114 marked as done.

[the1ts]

Fixed in the below commit.
http://github.com/the1ts/todo.txt-cli/commit/7baf05194211f5a67a8b9aff94955c2e096f38b7

Using the same regex used to find items for archiving, this tests if the item is marked as done before allowing the item to be marked as done.

[ginatrapani]

Merged, thank you!!