Bug 12357: Enhancements to RIS and BibTeX exporting

Bug 12357: Enhancements to RIS and BibTeX exporting

Some libraries would like to be able to add arbitrary fields to both the
RIS and BibTeX citation formats that a record can be saved as from the
staff intranet and public catalog. In addition, they would like to be
able to override the default record type and use Koha's itemtype as the
record type for those formats as well.

Test Plan:
 1) Apply this patch
 2) Run updatedatabase.pl
 3) Add the following to the new syspref RisExportAdditionalFields:
TY: 942$c
LC: 010$a
NT: [501$a, 505$g]
 4) Find or create a record with an 010$a (lccn) field, a 501$a field,
    a 942$c field, and multiple 505$g fields.
 5) Locate the record in the catalog, choose "Save" and select RIS
 6) Inspect the downloaded file, note the replaced TY field, the LC
    field, and multiple NT fields
 7) Add the following to the new syspref BibtexExportAdditionalFields:
'@': 942$c
lccn: 010$a
notes: [501$a, 505$g]
 9) Using the previously selected record, choose "Save" and select BIBTEX
10) Inspect the downloaded file, note the lccn, the multiple note
    fields, and the new record type value

Signed-off-by: Frederic Demians <f.demians@tamil.fr>
Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>