add support for use_sti_base_class option

[svoynow]

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.

[svoynow]

The reason this is failing on CI is that it doesn't seem like the CI build is running new migrations. My test involves a new model and therefore a migration.
If the migration is run the tests do pass.
Given that, anything we can do to facilitate getting this merged?