API Re-introduced ability to insert spam field before another existing field

I was updating an old site to use the new spam protector module, when I realised that the ability to specify the "insert before" field when adding the captcha field has been removed.

I've reintroduced this option with tests and documentation. :) To be careful, I've added a check to ensure the field you specify exists. The module will fallback to adding to the end if it doesn't.