The `--chapters` option is replaced with `--top-level-division` which allows
users to specify the type as which top-level headers should be output. Possible
values are `section` (the default), `chapter`, or `part`.
The formats LaTeX, ConTeXt, and Docbook allow `part` as top-level division, TEI
only allows to set the `type` attribute on `div` containers. The writers are
altered to respect this option in a sensible way.