Better support for preferred-citation

[graeme-a-stewart]

Many software packages would like to use the preferred-citation field to their CITATION.cff.

At the moment the initialiser treats this as an "Extra CFF field", which is not well supported, but I would argue that this is such a common use case that it should be better supported for editing the title, type, authors, identifiers, etc. It would also be great that for authors you could copy information from the main fields in the CITATION.cff as the overlap would usually be very high.