Introduce bash completions for kubecfg

completions only work for list, get, create, delete.
Technically this is already enough:

```shell
__kubecfg_q() {
  kubecfg 2>/dev/null "$@"
}
```

Since a non-zero return will be passed through the function call. Only difference is that if kubecfg exits $? (where $? is a number between 1 and 255) then that same error status will be passed to the caller instead of being squished into returning 1 for all errors but I'd say that's a "feature".

BTW, while the `2>/dev/null` in the middle of the command is valid, I'd say it's more usual to put it at the end of the command:

```shell
__kubecfg_q() {
  kubecfg "$@" 2>/dev/null
}
```

I still have some of a problem with this function that it doesn't really achieve much and just adding an explicit `2>/dev/null` elsewhere might not be too much more typing than the "__" and "_q" and the fact that you need to remember what it does... But if you still prefer with the function, that's fine with me.