Adding `citext` support.

[ringzhz]

Hi,

My project uses the citext plugin of postgres and I'd love to add support for it.
I have it working with my local DB with this simple change:

dave@zecarbon:~/ws/sequelize-auto$ git diff
diff --git a/src/auto-generator.ts b/src/auto-generator.ts
index 9d3b425..dd96730 100644
--- a/src/auto-generator.ts
+++ b/src/auto-generator.ts
@@ -807,7 +807,7 @@ export class AutoGenerator {
   }
 
   private isString(fieldType: string): boolean {
-    return /^(char|nchar|string|varying|varchar|nvarchar|text|longtext|mediumtext|tinytext|ntext|uuid|uniqueidentifier|date|time|inet|cidr|macaddr)/.test(fieldType);
+    return /^(char|nchar|string|varying|varchar|nvarchar|text|longtext|mediumtext|tinytext|ntext|uuid|uniqueidentifier|date|time|inet|cidr|macaddr|citext)/.test(fieldType);
   }
 
   private isArray(fieldType: string): boolean {

Would it be possible to add this?