Support non-default namespaces

[ahmetb]

I understand kubehost just shells out to kubectl. So technically user just do kubectl config set-context [...] --namespace=foo prior to running kubehost.

But it would be better if kubehost supported a native -n/--namespace argument and documented it.

[WilliamDenniss]

This would be nice to have. We also use the default zone, etc for all gcloud commands.