add support for use_sti_base_class option

This may be a very special case feature. But I don't think it clutters the code too much even if it is rarely used.
The idea is that if you're using STI, sometimes that's really an implementation detail and you don't want to have that reflected in your publicly facing IDs.
So if you pass this option, the generator use the row in the sequential table for the AR base class of the record.