Introduce bash completions for kubecfg

completions only work for list, get, create, delete.
Not sure if you can count on `_get_comp_words_by_ref` having been defined before this call...

I'm looking at `git` completion which uses it and I see they define their own if it's not present yet:
https://github.com/git/git/blob/master/contrib/completion/git-completion.bash#L157