[feature request] support for `--kube-context`

Typical helm commands let the user specify `--kube-context` for a specific context in the kubeconfig. 

There seems to be a workaround by specifying the environment variable `HELM_KUBECONTEXT` for helm under the hood (https://helm.sh/docs/helm/helm/), but having an explicit option in `helm diff` would be ideal and easier to discover.