Support module level docstrings

[philosofool]

I would like to add docstrings to my modules when working with nbdev, following PEP257 guidelines (afterall, the first way I look is just module? in Jupyter.)

The problem, for example:

I am exporting python code from a notebook example_maker.ipynb to a python file, example.py. When I do so, nbdev creates an __all__ statement at the top of the file. Adding an #export cell with a docstring to the first cell of example_maker.ipynb places the relevant 'docstring' below the __all__ statement that is autogenerated. This will not be recognized as a docstring and my linter also throws a no docstring error at me. Perhaps I have just missed how this functionality is supported via nbdev, but the examples in the documentation don't actually have module docstrings.

[jph00]

Working now in master. Here's an example module docstring cell:

#|export
_doc_ = """Read and write nbdev's `settings.ini file.
`get_config` is the main function for reading settings."""