feat: audit unloved should support explicitly unowned

[AndersDJohnson]

Hey @jjmschofield, thanks much for this awesome library! 💯 Are you currently accepting contributions? And if so, would you possibly have time to review, merge, and publish? If not, I understand - I'm happy to fork.

I'd love for github-codeowners audit --unloved to support the use case of explicitly marking some files as unowned. My use case is basically to enforce 100% CODEOWNERS coverage by ensuring this command has no output, but then allowing some known exceptions to be explicitly listed in the CODEOWNERS file.

GitHub appears to support this: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners#:~:text=owners%20are%20left%20empty

# In this example, @octocat owns any file in the /apps
# directory in the root of your repository except for the /apps/github
# subdirectory, as its owners are left empty.
/apps/ @octocat
/apps/github

Historically I'd been using @ghost entries, but GitHub's recent feature to present a UI for validation of files shows errors because that is not a real team.