LP#1367926: Expose non-MARC bre data via U2

LP#1367926: Expose non-MARC bre data via U2

Many use cases for the new unAPI implemetation depend on being able to run
recurring processes on new, updated, or deleted objects.  For most objects,
this is simple because they carry timestamps as fields.  However, the bre
class, as exposed by unAPI, does not provide access to many non-MARC fields.

So, here we add an 'includes' options to expose the non-MARC fields of use
to external processes.  The option is spelled "bre.extern", as in the tag
URI:

  tag::U2@bre/267{bre.extern}

Signed-off-by: Mike Rylander <mrylander@gmail.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Kathy Lussier <klussier@masslnc.org>