PermissionRequiredMixin: configurability

The check_permissions() method was using object, that is okay, but in case the ability to do something on a model rely on possessing some permission on another model that would result hard to deal with.
I think it's better to have another method to retrieve the permission object so that changing this behavior would not require the complete creation of another children mixin with the copy-paste of check_permission method
Ok done, sorry I did a fast edit on github