%config --persist magic

Quick and dirty thing on top of 
https://github.com/ipython/ipython/pull/4265 to have config magic write configuration to json file.
```
%config StoreMagics.autorestore=<Value|None> --persist <qtconsole|notebook>
```

Will write to `ipython_<qtconsole|notebook>_config.json`, `None` as value will remove the value from the file.