add owner loader

[zazayaya]

add owner loader decorator

example：

from flask_login import current_user

@casbin_enforcer.owner_loader
def owner():
    return [u.name for u in current_user.roles]

[hsluoyz]

@techoner @jessecooper @yyellowsun please review.

[jessecooper]

@zazayaya would it be possible to write some unit tests around this code?

[zazayaya]

@zazayaya would it be possible to write some unit tests around this code?

@jessecooper i have added owner loader unit tests, please review.

[jessecooper]

@zazayaya Thanks for adding the unit tests LGTM.

[jessecooper]

I will create and upload a new release to pypi as soon as possible.

[zazayaya]

I will create and upload a new release to pypi as soon as possible.

Thanks!!!