Add an option to insert a "signed off by" message into commit messages

[Caleb-T-Owens]

Some projects require users to include a sign off message at the end of their commits, in the format of Signed-off-by: Humpty Dumpty <[email protected]. Git can insert this for by creating it out of your git details if you pass a flag. It would be great if our new commit UI had an option to add this message on to the end of your messages for you