avoid n+1 queries in add_links when an association is eager loaded

This addresses the same problem as #98 
The difference is that if an association has not been eager loaded, it falls back to the pluck query so it will only load ids instead of full objects.
Probably the right thing to do is force eager loading of associations, rather than rely on the user passing a scope which eager loads them. However that's a little bit more work and I'm still getting familiar with this project.