Allow STDIN to kubecfg via `-c -`.

Follows the `tar xvf -` convention (because all the other conventions
are crazier?). Tried `--` at the end or a separate arg and all of them
felt worse.
This is going to say "Unable to read -:" because the comparison above was true. It might be better to say something human readable like "Unable to read from standard input" or "from stdin".