PermissionRequiredMixin doesn't allow custom permissions/object permissions

The PermissionRequiredMixin does not allow for checking custom object permissions provided by libraries like django object permissions or django-guardian.

I was thinking of updating it to be similar to https://github.com/lukaszb/django-guardian/blob/master/guardian/mixins.py#L52

Except stripping out the parts where it does object specific actions or guardian specific things, and allow overriding to implement said functionality.