Allow custom table join clause to override

Example : when we want to Include Campaign custom fields in contribution report, Just saying
protected $_customGroupExtends = array('Campaign');
won't help, since the joining clause would be different than usual.

With the PR : https://github.com/civicrm/civicrm-core/pull/2150
The contribution report could just say:
protected $_customGroupExtends = array('Campaign');
protected $customGroupExtendsJoin = array('Campaign' => 'contributioncivireport.campaign_id');