[feature] Be able to signoff a commit

[justinvyu]

Short description of the problem

I want to do the following within the vimagit CC commit step:

git commit -m "message" --signoff

It'd be nice to have a shortcut that just adds this at the end of my commit message:

Signed-off-by: Humpty Dumpty <humpty.dumpty@example.com>

Currently, I stage in vimagit and commit w/ signoff separately. If there's already a way to do this, please let me know!

Steps to reproduce

N/A

Environment

• vim or nvim version: nvim==0.7.2
• git version: git==2.32.1
• current vimagit version or (or SHA1): latest
• OS: MacOS
• terminal or gvim