feature: Support for footnote, as Github-Flavored-Markdown now supports footnote

[snawaz]

It seems footnote isn't supported yet.

https://github.blog/changelog/2021-09-30-footnotes-now-supported-in-markdown-fields/

[simonhaenisch]

Please raise this request in https://github.com/markedjs/marked or check if there's already an issue for it. This tool doesn't implement any markdown parsing by itself, it just uses Marked to convert it.