Add option to choose installation directory for `nbdev_install_hooks`

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.