Interactor registry

I like the idea of the interactors but for me as a developer it does not feel right to use a class name as an "interface" for something that essentially should be a method. So instead of `UserLogin.perform` I want to call a method `user_login`.

Here is the idea of the registry:

 ```ruby
registry = Interactor.registry
registry.user_login(email, password)
```
Use of dot notation for namespaces:

 ```ruby
module User
  class Login
     include Interactor
  end
  class Register
     include Interactor
  end
  module Admin
    class DeleteUser
        include Interactor
    end
  end
end

registry = Interactor.registry
registry.user.login # instead of User::Login.perform
registry.user.register # instead of User::Register.perform
registry.user.admin.delete_user # instead of User::Admin::DeleteUser.perform
```
You also have better control of indentation which makes your code more readable:

 ```ruby
module User
   module LongNamespaceName
       class DoSomethingWithAVeryLongName
          include Interactor
       end
   end
end

# without registry
User::LongNamespaceName::DoSomethingWithAVeryLongName.perform

# with registry
registry.user.long_namespace_name.
   do_something_with_a_very_long_name
```