`sub notebook` ability to leave notebook running and resume later

* Provide option to keep notebook running when you press `q`
* Re-attach when running `sub notebook`
   * start port-forward
   * launch browswer
* Future: Add ability for web ui to reconnect to running kernel and show the output

Use case: I have a lot of files downloaded in my running notebook that I wish to continue using even when I disconnect my browser accidentally