Configuration "load_as_reference" added

Add configuration key "load_as_reference" to support adding file as reference or import.
this will be useful for Maya for cases a Molder need to rework an asset, for that he will need to add file as import.

kwargs disclaimer and question:I try to avoid using kwargs, but in this case it seems the cleanest way. But I feel would be useful to pass the app object itself, so the hooks could have extra information.