Sometimes it's desirable to couple classes together by nesting. Currently there's no way to signal to an interface implementer that an inner class is a required attribute to be implemented. 

I propose a decorator for `abstractnestedclass` to fill this gap. A simple example use-case is outlined below:

```
from abc import ABCMeta, abstractnestedclass

class BaseComponent(metaclass=ABCMeta):
    @abstractnestedclass
    class Config:
        pass

class MissingNestedComponent(BaseComponent):
    # This will raise when constructed because 
    # BaseComponent.Config is not implemented
    pass

class NonraisingComponent(BaseComponent):
    # This will be constructable
    class NonraisingComponentConfig(BaseComponent.Config):
        pass
    pass
```