Add Features map option to DC:Coverage on other Omeka records

[jeremyboggs]

Omeka 2.0 includes Dublin Core metadata for Collections. We should consider adding the features map option to the collection's DC:Coverage field like we do for items. It might also be worthwhile seeing if its possible to detect whether there are other Omeka records registered in teh system, and whether they have a DC:Coverage field (or some other field) that might be enhanced by NeatlineFeatures.