Introduce bash completions for kubecfg

completions only work for list, get, create, delete.
I'm not sure I love the idea of looking for a function named after the command here, I think I'd rather have a `case ${command} in` and then call an appropriate function.

In the current case, that would mean you wouldn't have to define five functions that simply call `_kubecfg_service_match` or `_kubecfg_specific_service_match` but just have a single call to one of those inside two case statements, one that matches `get|delete|update)` and another that matches `list|create)`.

In the end, though, it's your choice. This code is fine and works as it is. Take this as a suggestion only.