Allow kubecfg to print custom types

Initialize HumanReadablePrinter with an array of print handler functions that can handle types other than base types.
IMO, HumanReadablePrinter and HumanReadablePrintHandler types ought to be combined. So the flow would be that you create a HumanReadablePrinter and then add handling functions to it. Look at how fuzzer's New() and Funcs() methods work: https://github.com/google/gofuzz/blob/master/fuzz.go#L40