ipc_gullwing_generator: add a way to specify the full output library name
requested to merge Kev-J/kicad-footprint-generator:gullwing-generator-override-library-name into master
As discussed with @cpresser here kicad-footprints!2754 (comment 836038527) , I added a way to specify the full name of the output library when generating footprints with the ipc_gullwing_generator.py.