Make the plugin compliant with authsplit one

OAuth options may be read through authsplit plugin.

Author: atisne

OAuthManager.php

@@ -253,7 +253,7 @@ protected function processUserData($userdata, $servicename)
                 $localUserInfo['grps'],
                 $userdata['grps'] ?? [],
                 array_keys($helper->listServices(false)),
-                $auth->getConf('overwrite-groups')
+                $auth->getOption('overwrite-groups')
             );
 
             // update user if changed
@@ -262,7 +262,7 @@ protected function processUserData($userdata, $servicename)
             if ($localUserInfo != $userdata && !isset($localUserInfo['protected'])) {
                 $auth->modifyUser($localUser, $userdata);
             }
-        } elseif (actionOK('register') || $auth->getConf('register-on-auth')) {
+        } elseif (actionOK('register') || $auth->getOption('register-on-auth')) {
             if (!$auth->registerOAuthUser($userdata, $servicename)) {
                 throw new Exception('generic create error');
             }

auth.php

@@ -195,6 +195,18 @@ public function getLang($id)
         return $parent->getLang($id);
     }
 
+    /**
+     * Get an option
+     *
+     * @param string $option The name of the wanted option
+     *
+     * @return string  The option value
+     */
+    public function getOption($option)
+    {
+        return $this->getConf($option);
+    }
+
     /**
      * Farmer plugin support
      *