Conversion of ipynb to/from py script

[chrisjsewell]

Principally for use with Visual Studio Code
and would also be an alternative to #1

vscode-python partially has this functionality, but currently does not support metadata import/export
(See Microsoft/vscode-python#4142)

Implementation:
- vscode-python/../jupyterImporter.ts
- vscode-python/../jupyterExporter.ts

When complete also added a documentation section

[stefanuddenberg]

I've been using jupytext in concert with ipypublish so that I can also have a pure (editable) markdown version of the notebook.

[chrisjsewell]

@stefanuddenberg and on this issue; I've implemented the export part of this here, and parsed on my code to the vscode guys. Yeh jupytext looks like it going for the same approach, and I really like being able to edit in a standard document and use all the intellisense and linting features that pycharm/vscode offer.
Both at the moment though they both lack being able to edit the metadata in the python document, which would be really helpful in terms of ipypublish.

[chrisjsewell]

I stand corrected, jupytext does output metadata, awesome!

I'll update the documentation with a suggestion of using it

[chrisjsewell]

Documentation updated with 608d936, ta @stefanuddenberg