Allow archetypes to run scripts

When a new entity is created from an Archetype we also want to have the ability to run scripts. These should be attached to the archetype and will act on the entity data only.
This is useful for doing things such as altering inherited properties (for example scaling the entity) or to randomize child entities (to allow for a random inventory when new entities are spawned for example).
Script hooks should be provided for both when the raw Atlas data is processes as well as when the entity has been created (before it's added to the world).