Implement git commit --coauthor="John Smith <js@something.com>"

From https://github.com/gitgitgadget/git/issues/343:

Since GitHub supports the Co-authored-by: trailer, I'd say it's a defacto standard.

Perhaps we could add an option such as git commit --coauthor="John Smith <js@something.com>" --coauthor="Smith John <sj@something.com>" which would automatically add the trailers. This should happen right before the SOB lines.

Things to consider:

• what other commands should get this option (anything supporting --signoff?)
• Do we want a config option for this? (Currently --signoff doesn't have one, does this deserve one?)