Allow multiple @Alias for a plugin

A plugin could have multiple alias. For example, the Help plugin could be invoked by "help" or "?".

This issue has two possible solutions :

1. Changing the @Alias value() method to return a String[] or
2. As Dan Allen suggested, introduce an @Aliases annotation that accepts a collection of @Alias annotations. this reads nicer and leaves the door open for per-alias customizations if we should need them.

The second approach looks nicer.