Feature request: Add the ability to signoff commits

**The feature request**
Add the ability to signoff commits, like in `git commit --signoff`, which adds this text to the commit:
```
    Signed-off-by: NAME <email>
```

**My use case**
Some repos require it.

**Implementation**
- [X] I would like to contribute this feature if it's a suitable addition to multi-gitter

Would you be willing to accept a PR that does that?
