Provide feature to add project authors to a file.

I've lately added the author-validation to my build tools. One thing I really miss is that only the files authors are recognized.

For the package files (e.g. `composer.json`) it would be great if all project authors are listed instead.

This pull request provides a new `scope` option to the command to switch to the project scope. Then the authors are compared to the project authors fetched from the git repository.

This pull request depends on https://github.com/contao-community-alliance/build-system-repository-git/pull/1