Bug 5600: Command line interface for tools/export.pl

Bug 5600: Command line interface for tools/export.pl

export.pl [--format=format] [--date=date] [--dont_export_items]
  [--deleted_barcodes] [--clean] --filename=outputfile

    * format is either 'xml' or 'marc' (default)
    * date should be entered as the 'dateformat' syspref is set
      (dd/mm/yyyy for metric, yyyy-mm-dd for iso, mm/dd/yyyy for us)
    * records exported are the ones that have been modified since 'date'
    * if --deleted_barcodes is used, a list of barcodes of items deleted
      since 'date' is produced (or from all deleted items if no date is
      specified)
    * --clean removes NSE/NSB

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>