Support extended taxonomies with built-in checklists

Simplest idea is to support a column in the .csv and field in the .btxm with a comma-delimited list of ISO codes (or really, eBird codes), which generates built-in checklists. That doesn’t give you rarity, introduced, extirpated status - that could be added with something like “US(R)” for rarity or “US(EX)”? Endemic would be “only one entry in the list”, I suppose.