* chore: Update help messages and adding examples.
This commit introduces fixes for the top-level help message as described in #187.
It does not address:
* #216 - Use `kn function` in help message when run as a plugin to kn
* #215 - Group main help message to put important commands to the top
* #214 - Make examples in usage message parameterizable
- Replaces globally-scoped formatter function with methods
- Defines enumerated Format types
- Renames the 'output' flag 'format' due to confusion with command file descriptors
- FunctionDescription now Function
- Global verbose flag replaced with config struct based value throughout