PASSWORD_FORM_FIELD set from django settings file

I'm using this with [django-two-factor-auth](https://github.com/Bouke/django-two-factor-auth) which provides it's own log in templates with different form fields. Thus, in addition to setting the `USERNAME_FORM_FIELD` from the django settings file, I also would like to set the `PASSWORD_FORM_FIELD` from the settings file.