Support signoff commit

[yu-iskw]

Please describe your suggestion: 提案の概要

Thank you for the great action.

We use the DCO app so that we accept only sign-off commits. When we manually commit changes, git commit --signoff enables us to create a sign-off commit. Whereas, when we create a commit with toc-generator, actions by the DCO app are always failed due to the lack of sign-off.

https://github.com/apps/dco

Describe the solution you'd like: 考えうる解決方法

I would like to add an inputs of the actions, say about signoff. If signoff is true, toc-generator create and push a sign-off commit.

For instance, the peter-evans/create-pull-request option offers the signoff input. We can control if a commit gets sign-off or not with the input.

• https://github.com/peter-evans/create-pull-request

Describe alternatives you've considered: 考えうる代替案

I don't have any alternative idea.

Additional context: 補足

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