Improve custom attributes in Django itself.

[surister]

Right now we have custom options at the Meta class, example:

class RefreshModel(CrateModel):
    field2 = models.TextField()

    class Meta:
        app_label = "test_app"
        auto_refresh = True

But we are doing Metaclass shenanigans to a accomplish this due lo the lack of extensibility of the django internals.

More details in https://forum.djangoproject.com/t/allow-meta-class-to-have-extra-database-specific-options/40650/3