CommandHandler: Call the SimpleCommandService/onCommandEvent listener di...

This is more to discuss, but it works so feel free to merge anyway if you think this is okay.

Sponge's SimpleCommandService [listens for CommandEvent](https://github.com/SpongePowered/SpongeAPI/blob/master/src/main/java/org/spongepowered/api/service/command/SimpleCommandService.java#L73) but *how should this be registered*?

The event manager requires a valid plugin instance, so should the Granite server be an implicit plugin by itself?

Right now it's just calling that event handler directly which works but is of course a *bit* of a hack :grin: