LaTeX template: set secnumdepth to 5 if `--number-sections` specified.

This yields behavior equivalent to the other writers, numbering
level 4 and 5 headers too.
ConTeXt writer: Removed `\placecontent`.

This produced a duplicate toc, in conjunction with `\placelist`.
Canonical way to set TOC depth

Use `\placecontent` instead of `\placelist` as `\placecontent` is easier to
configure.
Merge branch 'master' of git://github.com/jgm/pandoc-templates
Minor spacing tweak in context template.
Merge branch 'master' of git://github.com/jgm/pandoc-templates