add support for custom commit messages in action/manage-by-issue

It would be nice to have an input to override the commit message used by action/manage-by-issue, so the generated commit can follow a project's commit conventions (e.g. Conventional Commits).

example:

   - uses: mitchellh/vouch/action/manage-by-issue@v1
     with:
       commit-message: "chore(vouch): update VOUCHED contributors"

I tried a workaround using action/setup-vouch and calling the vouch command directly, but ran into an escaping bug in the wrapper that prevents passing a multi-word commit message: #94