Easier way to define index type for Postgres custom fields
Not sure what the purpose is for this change, can you explain?
 > Not sure what the purpose is for this change, can you explain?

Allow to deal with `index_type` without the need of inheriting from `IndexedFieldMixin`.
The simpler, the better :)
 I appreciate the simplicity of this... let me think about it. Thanks!

Thanks :)
