BibRecord: filter field instances

* Introduces new function for filtering field instances obtained
  from record.  Function returns only those field instances that
  match some-other-subfield/some-other-value criteria.  The function
  can work in 'exact', 'substring' and 'regex' mode.
  (addresses #1550)

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>
BibRecord: record_get_field_values with filtering

* Function record_get_field_values() now has additional
  parameters: filter_subfield_code, filter_subfield_value,
  filter_subfield_mode which allow to obtain only those
  field values that match certain filtering criteria.
  (closes #1550)

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>