Controls the `papersize` in ConTeXt template. Any valid argument of
`\setuppapersize[...]` descirbed on the [ConTeXt wiki][wiki] works.
[wiki]: http://wiki.contextgarden.net/PaperSetup
In ConTeXt, the canonical method to get unumbered sections is to use
\title {Unnumbered chapter}
\subject {Unnumbered section}
\subsubject {Unnumbered subsection}
etc. However, pandoc generates \section {...} tags irrespective of whether
`number-sections` is active or not. Personally I think that the default pandoc
output is better than generating `\title`, `\subject`, etc. in the output.
The current patch sets `number=no` for first four level of section heads when
`number-sections` is not set.
