Added format-specific exporter settings.

We didn't really need them for XML but they're going to be necessary
for MARC.
MARC export implementation

This feature uses the File_MARC PEAR package available at
http://pear.php.net/package/File_MARC/. Works reasonably well if you
know how to create the mapping and have some idea how MARC works. Still
have to implement getMappingErrors to check for issues with said
mapping.