Fix issue #361 - add persistence for priority on completion, as an option defaulting to original behavior

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[x] Fork the repository and create your branch from master.
[x] If you've added code that should be tested, add tests!
[x] Ensure the test suite passes.
[x] Format your code with ShellCheck.
[x] Include a human-readable description of what the pull request is trying to accomplish.
[x] Steps for the reviewer(s) on how they can manually QA the changes.
[x] Have a fixes #XX reference to the issue that this pull request fixes.

Reviewer, manual QA of change: Setup:

todo.sh add "(A) my task loses priority"
todo.sh add "(A) my task keeps priority"

Testing: Verify no change in existing functionality: Run:

@$ todo.sh do 1
1 x 2021-09-19 my task loses priority
TODO: 1 marked as done.

Verify that the Priority is removed, as is the currently expected behavior:

@$ todo.sh list
2 (A) my task keeps priority
1 x 2021-09-19 my task loses priority
--
TODO: 2 of 2 tasks shown

Verify that the new feature works (allow persistence of priority after completion, if user specifies -m): Run:

@$ todo.sh -m do 2
2 x 2021-09-19 (A) my task keeps priority
TODO: 2 marked as done.

Verify that the Priority persists after the completion:

@$ todo.sh list
2 x 2021-09-19 (A) my task keeps priority
1 x 2021-09-19 my task loses priority
--
TODO: 2 of 2 tasks shown

todotxt / todo.txt-cli