[Wip] Use Configurable for Nbconvert

Use configurable for nbconvert, 
Which should help to pass flags to custom converter without having to bring modification to the main file. 

I had to remove

```python
__metaclass__ = DocStringInheritor
```
As I can't figure out how to make it works with `Configurable`. I think @bfroehle wrote the this part... If he has any insight on how to do it...


Thanks.