Adding a mapping

[rcthomas]

What a nifty extension. I threw together a remote proxy handler but I now see #154, we'll probably just wait for that. I was wondering about a different kind of mapping like

/proxy/<slurm-job-id>/<port>/...

Or it could be anything really. Some logic would do the mapping between IP on the head node of a job running in a cluster but you'd want to keep that mapping in the handler. Is there a right way to keep that kind of state there? Anything to avoid/worry about?