Added possibility to config the IPython.nbconvert inside Nikola.

This simple PR makes the support from ipynb so "much" powerful because we have access to the IPython config system... so we can config a **LOT** of useful things.
One use case is the change of the template that `IPython.nbconvert` uses to render the ipynb to html... and there is a lot of more uses cases such as changing filters, preprocessor, etc... 
I will be great if we have this possibility in Nikola...

Working example of utility: visit this [post](http://www.damian.oquanta.info/posts/blogging-with-nikola-and-ipython.html) and click in any IPython input cell and... surprise ;-)

Cheers.

Damián