Small fix to create an empty Jupyter Notebook when the file is empty, or it doesn't exist at all.

[diegoortizmatajira]

• Added a new key in settings, allowing the user to provide their own function to generate an empty notebook.
• Added a default generator function that produces an empty notebook.
• Added the logic that checks if the file exists or, if it is empty to call the generator.
• Added a small description in the README file

[diegoortizmatajira]

Now it is possible to create a notebook by executing:

:e new_notebook.ipynb

or by creating an empty file with ipynb extension and then opening it.