Provide a way to include Co-Authored-By in commits

As pair programming is more and more common, it would be useful to be able to include co-authors in commit messages. The git community (openstack in this case) has agreed on using a trailer Co-Authored-By for that, but it is quite cumbersome to add that on every commit.

The Github Desktop tool added a feature which let you choose co-authors from existing project collaborators. This is a nice solution for this problem.