When calling `ein:jupyter-server-stop` ask to user if they are sure and also
give them the option to save any unsaved notebooks before killing the server.
Before closing the server also close any open notebook buffers.
For the moment we only support current version of jupyter (i.e. 4.3.1 or
greater).
The special commands are `ein:jupyter-server-start` to start a notebook server
and `ein:jupyter-server-stop` to, clearly, stop a notebook server.
On starting a server EIN will try to determine the url and token for accessing
the notebook server, login and open the notebook list automatically.
