Implement unity:prefabKeepObject

[fmoo]

Currently, using prefab imports nukes the SuperObject and the SuperCustomProperties components. While custom properties are broadcast, it may still be desirable to allow access to arbitrary custom properties by name at runtime.

Add a new unity attributes, unity:prefabKeepObject that on import, checks if set on the object, and if so, creates a copy of the SuperObject and SuperCustomProperties onto the root node of the prefab.