Expose deregistration attribute of proxy clients created by sens8

[geekofalltrades]

Unless I'm missing something, it doesn't seem to be possible to set the deregistration attribute of proxy clients created by sens8. I would like to set it to {"handlers": "deregister_client"}, so that when the proxy client deregisters (which is exposed, via deregister in the check definition), I can take action on that on the server.

If this is possible, then it isn't documented. :)