Fixes#2609.
This PR introduces the new-style section headings: `\section[my-header]{My Header}` -> `\section[title={My Header},reference={my-header}]`.
On top of this, the ConTeXt writer now supports the `--section-divs` option to write sections in the fenced style, with `\startsection` and `\stopsection`.
This is now the default for pandoc's Markdown.
It allows whitespace between the two parts of a
reference link: e.g.
[a] [b]
[b]: url
This is now forbidden by default.
Closes#2602.
