Add missing plugin.yml
Some improvements ;)

- Added javadoc to all CommandHandler methods
- Allow multiple identifiers in command annotations
- Moved annotations to separate packages
- Moved PermissionHandler and HelpHandler to separate package
- Allow adding a prefix when registering commands
- Allow overwriting argument handlers
- Moved default help handler and permission handler to extra classes
- Allow getting a list of all argument handlers
- Added boolean argument handler