Diesel support for Postgres citext Extension

This very small extension allows you to use the citext type with diesel, without having to make modifications to the schema.rs generated file. It takes much inspiration from diesel-postgis.

Before sending to postgres and after parsing from postgres, the text content is always passed through the Rust String to_lowercase method.

To Ensure citext in the schema.rs file is parsed correctly, add diesel_citext to the import types declaration in your diesel.toml file.

E.g. it will look like this:

[print_schema]
file = "src/schema.rs"

import_types = ["diesel::sql_types::*", "diesel_citext::sql_types::*"]