One small thing

[stjmtasts]

Hello,
You might have accidentally put a dot at the end of "GITHUB_USER.github.io**.**" in your usage guidance.
Other than that, I really appreciate your solution to this, you are a lifesaver!
Thanks.

[jimeh]

The dot is technically required, but most DNS providers no longer require you to add them and will instead silently do so without showing them. But must should be able to cope just fine with or without the dot :)