add support for OAuth state variable

Author: mmorrisson

src/Mailchimp.php

@@ -273,12 +273,14 @@ public function verifiedDomains($domain_name = null)
      *
      * @param $client_id
      * @param $redirect_uri
+     * @param $state
      *
      * @return string
      */
     public static function getAuthUrl(
         $client_id,
-        $redirect_uri
+        $redirect_uri,
+        $state = null
     ) {
         $encoded_uri = urlencode($redirect_uri);
 
@@ -287,6 +289,10 @@ public static function getAuthUrl(
         $authUrl .= "&redirect_uri=" . $encoded_uri;
         $authUrl .= "&response_type=code";
 
+        if ($state !== null) {
+            $authUrl .= "&state=" . $state;
+        }
+
         return $authUrl;
     }