Add custom support for opening a terminal and running a specified command

[dhirschfeld]

The first part is easy with the jupyterlab-commands type:

  type: jupyterlab-commands
  source:
    - label: Open Terminal
      id: 'terminal:open'
      args:
        name: Browsr

...but it appears that there is no way to use that to execute a command (e.g. launch a terminal app) for security reasons:
jupyterlab/jupyterlab#5916

initialCommand is still a constructor arg so jupyterlab_app_launcher could define a special terminal type and create the terminal and run the initial command via the typescript api.

[trungleduc]

Hi, thanks for the suggestion, i'm happy to review PR implementing this feature