Add "config" command to manage configuration file

### Feature request

Add a new command group `config` for managing `kn` configuration files

### Use case

`kn` can be customized by a local configuration file, that e.g. can specify how plugins are found or add additional sink mappings.

In order to provide an enhanced user experience to manages that configuration a new top-level command group, `config` is proposed.
The details of the subcommand still need to be fleshed out, but I could imagine having commands like:

```
# Create a default configuration file, with annotated comments for possible options
kn config init

# Set a configuration option for scalar options with scalar values. Key specified as JSON path
kn config set plugins.directory=~/.kn

# Get an individual configuration value or group of values
kn config get eventing

# Show the whole configuration
kn config show

# Specialized commands for specific use cases that involve list commands
kn config sink-mapping add svc --group core --version v1 --resource services
kn config sink-mapping delete svc
kn config sink-mapping list
```