Unfortunately this will need to be partly redone, since retrieveItem(), and
therefore itemToCSLJSON(), and therefore itemToExportFormat(), need to be
synchronous. The item data load statements in itemToExportFormat() will
probably need to be performed earlier, when they can be async, and made
available to the session for retrieval by retrieveItem(), but I'll let someone
more familiar with the citation infrastructure do that.
This restores some code in retrieveItem() that may have been accidentally
removed in a merge, though it probably won't be useful anymore anyway.
Addresses #529
