Add option to choose installation directory for `nbdev_install_hooks`

[thomasht86]

When trying to run nbdev_install_hooks, I get

PermissionError: [Errno 13] Permission denied: '/home/azureuser/.jupyter/jupyter_notebook_config.py'

The reason is that I work in an Azure Machine Learning workspace, where no users have access to their home directory.
I think this could open up nbdev to a large user-base that really needs it. 🙏

Proposed solution:

• Add hook_install_path option to the settings.ini-file.

[seeM]

Where do you store your own jupyter_notebook_config.py? Do you use Jupyter-specific environment variable(s) to set that? I'd prefer if we made nbdev_install_hooks use the same environment variables instead of adding another setting.