Update core/components/groupeletters/processors/mgr/groups/list.php

Added option "returnJSON". (Opt) Returns a JSON encoded object of results. This is useful in the event that you need to manipulate the returned data in a snippet. [default=]

Example (this is inside a snippet):
```php
$list = $modx->runSnippet('getResources', array(
  'parents'=>'119',
  'depth'=>'3',
  'limit'=>'5',
  'returnJSON'=>true
));
//$list is now a json string
$list=json_decode($list);
//$list is now a php object
print_r($list); //Array ( [0] => stdClass Object ( [conditionalTpls] => [context] => [dbCacheFlag] => 0 [debug] => [depth] => ...
```