Add support for show command to render non-K8s objects

[wojciechka]

Currently kubecfg show can only be used to render Kubernetes objects. This makes sense given the origin of kubecfg and its ties to Kubernetes.

For example:

$ echo '[{ hi: "hello" }]' >/tmp/test.jsonnet ; jsonnet /tmp/test.jsonnet ; kubecfg show /tmp/test.jsonnet
[
   {
      "hi": "hello"
   }
]
ERROR Error reading /tmp/test.jsonnet: Looking for kubernetes object at <top>[0].hi, but instead found string

Being a generic tool with certain properties, kubecfg show could also be used to render generic JSON or YAML files.

One option is for kubecfg show to include new flag that disables the validation - such as kubecfg show --validate=false or --kubernetes-objects=false, but this is just a suggestion and I am open to other ways in which this would get implemented and/or discussion about it.