Trigger JupyterLab extension from a command in the notebook

[edgano]

The idea is the possibility to trigger an extension (HTML viewer) when the user write something like open http://

We talk on gitter and @ellisonbg propose a mime render to take the JSON from the kernel.

With what I read before, my idea was:

• Identify the command sintax (open http) in the kernel
• Send a message in IOPub from the kernel
• Capture the msg in JupyterLab and trigger the extension