Add `Signed-off-by` trailers to commit messages

[emersion]

Is your feature request related to a problem? Please describe.

My organization requires all commits to have a Signed-off-by trailer for DCO.

Describe the solution you'd like
Add some way (e.g. configuration option) to enable Signed-off-by trailers. Their content is the same as the commit author.

Describe alternatives you've considered

• Auto-discover the GitHub signoff policy, but it doesn't seem like there is a way to query this setting via the API.
• Add templates for commit descriptions in addition to commit titles.

[martinjagodic]

@emersion would be open to providing a PR for this feature?

[emersion]

Absolutely!