Use the copyright holder in check

I would like to use addlicense as linter in projects like [capa](https://github.com/mandiant/capa) and [FLARE-VM](https://github.com/mandiant/flare-vm) to ensure all files have a header with `Google LLC` as copyright holder with `addlicense -c "Google LLC" --check .`. But at the moment it seems addlicense ignores the `-c` argument if the `-check` argument is provided. I would like that if both arguments are provided, it checks not only that there is a header, but also that the copyright holder is correct.

Start by reproducing the reported `addlicense -c "Google LLC" --check .` behavior and trace how the check mode handles the copyright-holder option. Done means check mode verifies both that a license header exists and that its copyright holder matches the value supplied with `-c`.

Written by the indexing model from the issue text.