GitHub integration: close linked Nolt-GitHub issues automatically as soon as they are closed in GitHub#318

This would be rather beneficial for us as right now we need to go to remember which Nolt issues have been closed with closing certain GitHub issues. It would be much easier if once the GitHub issues created with GitHub-Nolt integration (which is amazing btw (!)) get closed, the linked Nolt issues would get closed too.

Thanks, Nikita. I can definitely see how this can be useful.