* Adds zsh completion
- Adds back zsh completion after vendoring cobra library with a fix
- Updates the command definition to accept argument either of ['bash', 'zsh']
- Moves the completion command definition under `pkg/kn/commands/completion`
- Updates docs
- Updates CHANGELOG.adoc
* fix(golint): exported function should have comment
* Fixes PR number in CHANGELOG.adoc
* Updates unit tests
Navid Shaikh