Add boolean template function 'namespaced'

[reegnz]

Example usage:

kubectl-slice -f test.yaml -o test --prune -t '{{ if namespaced .}}namespaces/{{.metadata.namespace}}{{else}}cluster{{end}}/{{.kind}}.{{.metadata.name}}.yaml'

[reegnz]

This is just an early draft to get some ideas flowing for #133.

Doesn't yet contain external config loading logic for looking up namespaced vs cluster scoped resources.