Insert the completion date once a task is completed

[fnuttens]

It should be inserted by default, but there should be a deactivation flag with the do command, such as do 12 --no-completion-date.

⚠️ this should fail if the completed task doesn't have a creation date, as specified in the todotxt specification.

To summarize, a completed task should (by default) look like the following:

x (B) 2020-05-18 2020-05-16 create my first issue +todotxt-rs