feat(derive): Add nested opt

[jungnitz]

Hi Ben,

thank you for this crate! In my project I have some Partial structs with fields that are Partial as well. Therefore, I implemented a way to use the Partial::Item type in the generated struct and call apply_some on the field in the implementation via a new nested attribute.

[bengreenier]

Thanks @jungnitz this looks neat - I need to review it properly, but will hopefully have time for that this weekend.