Add option to ipynb2jupytext to convert only code cells

[fecet]

As an avid user of Colab and Kaggle, I often come across tutorial notebooks that are filled with HTML, pictures, and Markdown content. While these elements can be helpful for learning, they can also be distracting and unnecessary when I want to run the code on my own server. Additionally, I still read the notebooks on the Colab or Kaggle pages even after I've converted them to Jupytext format.

To address this issue, I propose adding an option to ipynb2jupytext that converts only the code cells to .py format. This would make it easier to work with the code on my own machine without having to sift through the extraneous content.

[kiyoon]

Sounds like something everyone might need at some point. Thank you!