Improve .zshrc to display github information when I'm in a github-hosted repo

[phargogh]

My current use case is that if I'm in a local repo called invest, is that my fork or is it the upstream repo?

I'd like to add a line to my zsh prompt, something like this:

[gh]phargogh/invest forked from natcap/invest
(usual prompt here)

I think ideally, and if the terminal supports it, it would be nice if [gh] were the github logo and I used the fork icon to cut down on words used.