Follow #434 when clobbering an AN import

* `replace` deletes all existing sections with the same heading (or
  no heading) as the one being imported. If there are more sections
  with that name in the import, all of them are imported.

* `merge` if there is an existing section with the same heading as
  the one being imported, speeches of all sections in the import
  with that heading will be added to it (if there are more existing
  sections with the same name, speeches are added to the first one).
  If there is no existing section with the same heading as the one
  being imported, all sections with that heading are imported as
  separate sections.

* `skip` if there is an existing section with the same heading as
  the one being imported, import of the section is skipped