Modified `ein:jupyter-server-use-subcommand` to be a list.

[jeeger]

This allows using more complex subcommands. For example, launching jupyter from poetry can be done by setting ein:jupyter-server-command to "poetry" and ein:jupyter-server-use-subcommand to '("run" "jupyter" "lab")`.

[dickmao]

There are zero to two guys out there whose init.el contains ,

(custom-set-variables '(ein:jupyter-server-use-subcommand "foo"))

which would collide with logic that assumes that variable is now always a list.

With ac92eb92 perhaps try setting the variable to "run jupyter lab".