Option(s) to control channel access

[parente]

Option to limit access to one or more Jupyter protocol channels: shell, iopub, control. Maybe to limit to specific comm channel names within shell too.

Use case: Restricting kernel access to listen-only.

[rgbkrk]

Listen only is a good case for iopub and using SSE instead of websockets.

[parente]

I think supporting more than websocket alone is going to be one of the more interesting aspects of the gateway. I chatted with some folks at Jupyter Day about the idea of combining the idea in #10 with, say, the ability to front specific notebook content (functions? cells?) with a pure HTTP API. Kernels then become little microservice engines.

At this point, I don't mind lumping a lot of different gateway features in the ref impl in this repo. Part of "graduation" will be figuring out if things should be refactored into separate projects.