Add co-authors to list of contributors

[JeanMertz]

We should parse any trailers and if there is a Co-Authored-By field, the co-author should also be added to the list of release contributors.

The supported syntax is the same as is already used elsewhere, including by GitHub, GitLab and as referenced by the Git wiki itself:

Co-authored-by: name <[email protected]>
Co-authored-by: another-name <[email protected]>"

There are others that we could/should probably support out-of-the-box as well:

Signed-off-by
Reported-by
Tested-by
Reviewed-by
Suggested-by
Acked-by

This is blocked on rustic-games/conventional-commit#9.