Feature Request: Parse Markdown in Footnotes

Thank you for your work on Cybermonday! It's a great library!

And thank you for including the footnote extension. I have a feature request for footnotes, which is that the text inside a footnote would also be parsed as markdown.

Current behavior:

```clj
(require 'cybermonday.core)
(cybermonday.core/parse-md "Markdown body[^1]\n[^1:My note with `code here`]")
{:frontmatter nil, :body [:div {} [:p {} "Markdown body" [:sup {:id "fnref-1"} [:a {:href "#fn-1"}]] "\n" [:sup {:id "fnref-1:My note with `code here`"} [:a {:href "#fn-1:My note with `code here`"}]]]]}
```

Desired change in output:

```clj
{:frontmatter nil, :body [:div {} [:p {} "Markdown body" [:sup {:id "fnref-1"} [:a {:href "#fn-1"}]] "\n" [:sup {:id "fnref-1:My note with `code here`"} [:a {:href "#fn-1:My note with " [:code {} "code here"]}]]]]}
```

Notably, now it includes: `[:code {} "code here"]` in the footnote.

I'm a Clojure newb, but I would be willing to give a PR a shot if you'd be open to feature and believe the change would fit with your current implementation. Thanks!