Support custom parameters to start-notebook.sh

## Motivation
In the [jupyter-docker-stacks Docs](https://jupyter-docker-stacks.readthedocs.io/en/latest/using/common.html#ssl-certificates), the proposed solution for HTTPS connection to the notebook is to mount the certificates via volume (this we can already do via `custom mounts` config :tada:) and passing extra config parameters to the `start-notebook.sh` commands.

```console
docker run -it --rm -p 8888:8888 \
-v /some/host/folder:/etc/ssl/notebook \
jupyter/base-notebook \
start-notebook.sh \
--NotebookApp.keyfile=/etc/ssl/notebook/notebook.key \
--NotebookApp.certfile=/etc/ssl/notebook/notebook.crt
```

There are other Jupyter config options that might be useful in this way, e.g. setting up a password

https://jupyter-docker-stacks.readthedocs.io/en/latest/using/common.html#jupyter-server-options

## Proposal
Add a new configuration option, perhaps titles `JUPYTER_SERVER_OPTIONS`, by default empty, and pass it on when starting a container.

If this all sound okay to you @csadorf I can take this on.