Feature request: footnote support

[hannahilea]

Thanks very much for this package, it has been very useful!

How challenging would it be to support footnotes, as does Github-flavored markdown?

e.g.,

Footnotes are added like so[^fun]. Here's another sentence[^rad].

[^fun]: Congrats, here is a footnote!
[^rad]: This too!

Footnotes are added like this[^fun]. Here's another sentence[^rad].

[^fun]: Congrats, here is a footnote! [^rad]: This too!