Footnote support?

[GitHub supports footnotes](https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#footnotes), but it seems they have not published [an updated spec](https://github.com/github/cmark-gfm/issues/288). 

Is footnote support something you would be willing to include in your extension?