separate the `$knctl help` to group commands for easier readability

[maximilien]

#nicetohave

[cppforlife]

implemented in f59f8b0

currently looks like this:

Basic Commands:
  curl              Curl service
  deploy            Deploy service
  logs              Print logs
  pod               Pod (list)
  revision          Revision (annotate, delete, list, tag, untag)
  service           Service (annotate, delete, list, open)

Build Management Commands:
  build             Build (create, delete, list)

Secret Management Commands:
  basic-auth-secret Basic auth secret (create)
  service-account   Service account (create)
  ssh-auth-secret   SSH auth secret (create)

Route Management Commands:
  domain            Domain (create, list)
  ingress           Ingress (list)
  route             Route (create, delete, list)

Other Commands:
  namespace         Namespace (create)

System Commands:
  install           Install Knative and Istio
  uninstall         Uninstall Knative and Istio
  version           Print client version

Available Commands:
  help              Help about any command

should figure out what to do with 'other' vs 'available' for remaining commands.

[cppforlife]

shipped in 0.0.8