Making diesel optional

[LucaCappelletti94]

Hi @vitaly-m and thank you for making this crate. I am using it in a monolithic web application, where I use Rust for both the yew-based frontend and diesel-based backend. As such, I define the structs associated to the tables once in a common crate which is shared for both frontend and backend, with different feature guards where needed.

Since I'd like to avoid duplicating the postgis-diesel structs, I would find it best if diesel was a default but optional feature of guarded by a feature.

I will be making a pull request adding such feature guards shortly, hopefully I made the reason for this change clear.