Introduce bash completions for kubecfg

completions only work for list, get, create, delete.
Although I can't think of who would do it, it's conceivable that someone might want to enable completions for kubecfg only and not for all bash-completions. I'd have a slight preference for making this file a closure where everything that's used gets defined here, but I can understand if you really resist duplicating that function here. (But, in that case, why are you OK with duplicating `__contains_word` which is also defined elsewhere?)