support diff with offline server state

[rbtcollins]

Ran into this working with kops kubecfg; the 'server' for kops isn't understood by kubecfg, but I could work around that iff 'diff' --diff-strategy=subset was able to operate on a local YAML representing the server state, rather than always connecting to k8s.

This would also be useful in some places with k8s itself, I suspect.