Support other filetypes than ipynb

[Johan-Noren]

In addition to creating ipynb-files i often find myself needing to create raw scripts (.py-files) in jupyterlab.

So it would be nice if i could add .py-files (or .scala/.R/.jl files for that matter) alongside the ipynb-files in the template-directory and the extension would create new files with choosen template and with correct file extension. The correct file extension would be determined by and copied from the template file extension.

[bluss]

Would be useful for jupytext (.py format) style notebooks