Add a feature to commit only source code(.py)

[iaoiui]

There are some needs to commit only source code but ipynb.
Hence I add a feature to commit only source code by checkbox.

thanks

[psychemedia]

I notice that this PR used nbconvert. Jupytext has also come on a long way over the last year or so, and provides a range of options for converting from ipynb to a range of other text document formats (py, md etc), although with a loss of code cell output during the conversion process.