Introduce bash completions for kubecfg

completions only work for list, get, create, delete.
You can drop the `tail` and use `awk` to skip the 3 first lines. You should probably also test whether the kubecfg command succeeded before proceeding. I'm not really sure that the __kubecfg_q macro is helping a lot...

If you want to keep it, maybe use a kube::completion::* namespace for the internal functions in here? (e.g. kube::completion::kubecfg_quiet). But then it's probably too long to be useful to shorten the script...

```shell
  local kubecfg_output
  if kubecfg_output=$(kubecfg list pods 2>/dev/null); then
    local pods=($(echo "${kubecfg_output}" | awk 'NR > 3 { print "pods/" $1 }'))
    COMPREPLY=( $( compgen -W "${pods[*]}" -- "$cur" ) )
  fi
```