Renaming must overwrite the existing context name

[ahmetb]

If there's a context named test, this will fail today:

 kubectx test=gke_ahmetb-samples-playground_us-central1-b_test

I think we should overwrite the existing entry, but the important point here is what to do with users and clusters entry of that context that we'll be deleting. This is to be decided in #23.