Add black notebooks functionality

[Bultako]

This PR is a first attempt to address issue #1756

black_notebooks.py script formats code in notebooks using black Python code formatter.
https://github.com/ambv/black

One issues still not solved is that cells containing %magic commands cannot be formatted.

[cdeil]

@Bultako - Thanks!

I tried to find the function / method in IPython or Jupyter that would allow us to split the cell content into lines with % or %% or !, i.e. non Python content, and still apply black to the rest. There's this but I didn't find a working example or try yet. Maybe we should just do something link if '%' in line and only pass the other lines to black?

I'll merge this now and will try it out in master.

[Bultako]

%magic commands exceptions solved in e0851fc