Add hook_microdata_value_type_alter

Currently, if we don't recognize the property type, we assume the field represents an item. However, most fields that use the Entity Property API are actually structs, meaningless containers for the more interesting bits of data.

Microdata should assume that if it doesn't recognize a property type that it is a struct. However, there are some fields (such as addressfield) which need to override this assumption. An alter hook would allow modules to do this.