pick up default namespace from .kube/config instead of hardcoding it in kn

[cppforlife]

[rhuss]

Maybe also add a global --namespace option for specifying the namespace to operate on.

[maximilien]

I like @rhuss suggestion.

@cppforlife can you elaborate on your title? Or is it simply reading the current ~/.kube/config and getting the namespace from the first context section. So in my case the namespace default from:

apiVersion: v1
clusters:
- cluster:
    certificate-authority: [redacted]
    server: [redacted]
  name: knative-demo
contexts:
- context:
    cluster: knative-demo
    namespace: default #<---- this namespace
    user: maxim@us.ibm.com
  name: knative-demo
current-context: knative-demo
kind: Config
...

[rhuss]

Related to #167 I guess.