Handle edge case in dontprintZoteroItems()

If a Zotero item has more than one attached PDF files, then
dontprinting that item used to be handled incorrectly: Dontprint
used to always choose the first attachment only, even if the
user explicitly selected one or more different attachments.
This made it impossible to dontprint other attachments than the
first one of any Zotero item, e.g., specific chapters of a book.

This commit fixes this issue. We still choose the first attachment
if the user selects an *item* with more than one attachments.
However, if the user specifically selects one or more attachments
of an item, then we dontprint those. If both the item itself and
one or more specific attachments were selected, then we also
dontprint only the selected attachments.

Fixes #8.