Allow use of --number-sections

[phiresky]

It would be good to be able to number the sections. Pandoc has an option for this, but it's not directly usable.

Changes in rootstock:

• Authors, Abstract, Bibliography should be {.unnumbered}
• number-sections: true in default.yaml

Changes in pandoc itself:

This probably needs fixing of jgm/pandoc#5071 since otherwise with the recommended level-two headings (or with shift-heading-level-by: 1) the numbers are wrong.

[dhimmel]

Thanks for sharing this. Numbered sections were mentioned by @agitter in #365 (comment).

I think greenelab/covid19-review#207 is a helpful reference.

Perhaps we should add a note in USAGE.md about numbering sections?