Support for citext

This is less intrusive now, postgresql-simple added CI instances for Text. Since case-insensitive has bytestring instances that assume encoding (and latin1 at that) Leon decided to not provide a polymorphic instance.

It's a free dependency. It does require the latest version of postgresql-simple, if we care about backwards compatibility I can CPP it.

And as previously discussed, citext isn't necessarily available on all postgres installs, but it is a standard extension.