Add tab completion for resource names

Example: `doctl kubernetes cluster kubeconfig save [tab]` would suggest existing clusters to tab complete.

`kubectl` does this for all existing resources. Looks like the implementation is [here](https://github.com/kubernetes/kubernetes/blob/f2469a7a2c75a78b8c0b0514652511d105d2b2e7/pkg/kubectl/cmd/cmd.go#L155-L166).

Figuring out how this works should only be difficult for the first resource we implement it on.