Added github_delete.py to delete student repos

[seemong]

Per https://education.github.community/t/clean-up-student-repositories-from-organization/48457, it seems like lots of professors want to delete student repos with a simple tool. As your comment said, it was a simple change in your set of excellent utilities. I copied github_clone.py and added the functionality to delete all repos with a subprocess call to the gh CLI (which needs to be installed beforehand).

[danwallach]

Anyway, if you want to make all these changes, I'd love to take a PR from you. While you're at it, don't forget to add documentation to the README.md file. Thanks!