introduce `skip_insertion` to skip a field for `#[derive(Insertable)]`

[nnbyte]

As I understand it, it is now possible in Diesel to skip_insertion that would allow to skip a field for #[derive(Insertable)]. Also, check out the diesel's rustdoc.

What I understood from Diesel's author himself, is that it is a very common misconception that is best practice to generate separate "insert structs"!

Hence my kind request for you to introduce skip_insertion so that it is not necessary to have separate "insert structs" anymore and thus simplify the resulting generated code (i.e 1 struct for all CRUD tasks).

Thanks a lot!