Support CITEXT type

[cocoliliace]

tokio-postgres added support for the CITEXT type in this commit: rust-postgres/rust-postgres@02fdb06. I think it would be nice to have it in cornucopia too :)

Edit: I had read #2 but I misread it the first time and thought it missed CITEXT. After re-reading it I realized it was talking about how CITEXT cannot be supported, so I'm closing this.

[cocoliliace]

Actually, I think I read it correctly the first time, CITEXT is supported in FromSql and ToSql now, so maybe cornucopia can support it?