Add support for custom finder as per cake 3.1

[birdy247]

Since cake 3.1 it is suggested to use a custom find method to modify the query to find a user (as opposed to Scope)

http://book.cakephp.org/3.0/en/controllers/components/authentication.html#customizing-find-query

It would be great if this plugin could utilise this functionality.

Thanks

[ADmad]

Make a PR :)

[ceeram]

Closing as no solution has been provided and the maintainers no longer actively maintain the cake3 branch, see also: #58