Added COMMAND_INITIALIZE event

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | tba

This event fires before the initialize() method is called on the command, enabling listeners to validate the input prior to execution.