add some convenience to loading default config

These are the pieces necessary for an object to get its default config from the global application if there is one, with:

```python
from IPython.config import get_config, SingletonConfigurable

class MyObject(SingletonConfigurable):
    def _config_default(self):
        return get_config()
```

I am using this in a patch to lib.latextools, which I will open as a separate PR.