Add TODOTXT_VERBOSE to the configuration

[inkarkat]

There's no command-line option to reduce verbosity (just -v to increase it), so users who would like to remove the additional messages (cp. https://github.com/todotxt/todo.txt-cli/discussions/364) have to configure this, but the variable is hard to find. Include the default value in commented-out form and some documentation of the possible values.

• [X] Fork the repository and create your branch from master.
• [ ] 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.
• [ ] Steps for the reviewer(s) on how they can manually QA the changes.
• [ ] Have a fixes #XX reference to the issue that this pull request fixes.