Allow to use role classes, not only blocks

[davydovanton]

We need to add ability to extract role logic to classes. Something like this:

module Post
  module Roles
    class Anonymous
      def call(user, _)
        user.id.nil?
      end
    end
  end

  class AnonymousAbilities
    include Kan::Abilities

    role :anonymous, Roles::Anonymous

    register(:read, :edit, :delete) { false }
  end
end

[davydovanton]

completed by https://github.com/davydovanton/kan/pull/18