Auto numbering for headings

[simonhaenisch]

Could be done purely with CSS (see http://2ality.com/2012/01/numbering-headingshtml.html), but would require extra styles for the TOC (if that get's implemented). Alternatively, it could be done with a custom renderer (see markedjs/marked#334 (comment)), which would be more flexible regarding styling (e. g. font-feature-settings: "tnum"; font-variant-numeric: tabular-nums; and better alignment for the numbering in the TOC).

[hadrien-toma]

I also prefer the "with a custom renderer" way of accomplishing it, this feature would be awesome ❤️.