Make the `backend` argument required in the `get_extractor` internal
routine. Specify a backend in the scripts which call this function. Add
a CLI backend option in capa/features/freeze.py as well.
Make the `backend` argument required in the `get_extractor` internal
routine. Specify a backend in the scripts which call this function. Add
a CLI backend option in capa/features/freeze.py as well.