Clobbering refinement in AkomaNtoso import

Clobbering works on all levels of section hierarchy now, not on the
top-level section only. There are three options for clobbering of
sections with the same heading:

* `skip` to skip import of sections that already exist,
* `replace` to replace existing sections with the imported ones and
* `merge` to append content of the imported sections to the existing
  ones.