Add Flask.config_class attribute

Similar to ``request_class``, ``response_class`` and ``app_ctx_globals_class``, the ``config_class`` attribute of ``Flask`` allows to specify custom class that stores the configuration for the app:

```python
import flask

class Config(flask.Config):
    def __getitem__(self, key):
        try:
            return super(Config, self).__getitem__(key)
        except KeyError:
            return None

class Flask(flask.Flask):
    config_class = Config
```

The current alternative is to override ``make_config`` and repeat its logic (such as handling of ``instance_relative`` argument). This should be more convenient and robust.