[WIP] Export markdown cells as block strings instead of comments

[tonyfast]

This pull request changes how markdown cells are written from the Python exporter. Currently, nbconvert creates Python comments which are not captured by the ast. This change wraps markdown cells in Python block quotes which are included in the ast.

The advantage of this change is that the first markdown cell may be used as both a module docstring and doctest.

[mpacer]

This is neat. @takluyver do you see any side effects with this?

Could you explain why your example includes assert True?

Could you explain why your example includes jupyter nbconvert --version?

I am asking because I think they are questions that should be answered in the docs accompanying this feature if it is merged. People will benefit from a step by step explanation of what is happening so they can start to think about other ways this idea could be explored.

[tonyfast]

Thanks for having a look at this.

Could you explain why your example includes assert True?

>>> assert True is a doctest, I was just using it aslogic to prove that a doctest was indeed captured from the first markdown cell.

Could you explain why your example includes jupyter nbconvert --version?

This is just a sanity check for the example.

[takluyver]

I don't see anything obviously wrong with that technically, though I imagine that there'll be a fairly even split of people who like it and people who don't.

What might be neat is to do it with just the first cell (if that's markdown), and convert the rest as ordinary comments. Unfortunately I think that's trickier to do in the template system.