Footnote, TOC Support

[Brin-o]

Describe your feature request I've been testing out iA Writer a little bit I prefer FS Notes over it, however there are some additional markdown features it has (I think it's from MultiMarkdown) that I ended up quite liking.

These features are the ability to add footnotes and TOC Support. The syntax for them is {{TOC}} to generate a TOC and word[^1] [^1]footnote comment later in the page to generate a footnote.

These are fantastic additions especially for academic writing where both TOCs and footnotes are common.

I am not sure what your stance is on markdown processing and if you'd be willing to stray from vanilla markdown so sorry if this is out of the question for you!