Support `kubernaut env` to emit useful environment variables

[ark3]

Make it easy to set up access to the Kubernaut facilities made available by the current claim. Something like

$ kubernaut env
KUBECONFIG=/Users/ark3/.kube/kubernaut
KUBERNAUT_IP=18.72.0.100
$ eval $(kubernaut env)

Future Kubernaut features could add to that list of variables, e.g.,

KUBERNAUT_DOCKER_REGISTRY=18.72.0.101

It might be useful to support other shells (kubernaut env --shell=fish). Presumably once multiple claims are a thing, this subcommand would need a --name flag as well.