Add support for case insensitive text columns (citext)

[matthewmcgarvey]

Fixes #550

Usage

In a migration you'd pass the the case_sensitive: false flag to a string column to make its type citext. The citext extension must be enabled.

enable_extension "citext"

create table_for(User) do
  add name : String
  add email : String, case_sensitive: false
end

This would create the users table with a case sensitive name column and a case insensitive email column.

[jwoertink]

Wow, this is pretty awesome. I had never even heard of this before. I kinda want to switch my columns over to it. I assume this works on alter table as well, right?

[matthewmcgarvey]

Yep, pretty sure we splat any extra options onto the type so it will get passed along. 🎉