Introduce bash completions for kubecfg

completions only work for list, get, create, delete.
I'm leaning towards not shipping it and counting on the one from bash-completion, since I'm worried that shipping a stoopid one might interfere with others that need it if they also just rely on another that's already included...

If you're keeping this, then at least use `declare -F` instead of `type` to test for the existence of the function, since that will work with functions only and not other types of callables.