Add hook to CRM_Utils_System::redirect

Proposal to add a hook into the redirect function. This is consistent with Drupal which has a redirect hook. 

The specific use case is that the postProcess hook of the Merge page redirects to a listing page. This listing page only calls a hook (pageRun) AFTER the various processing has been done. In order to alter a parameter for the page I think the most logical place is when the redirect is being processed

```

/**

• Redirect to another URL.
  *
• @param string $url
• The URL to provide to the browser via the Location header.
  */
  public static function redirect($url = NULL) {
  if (!$url) {
  $url = self::url('civicrm/dashboard', 'reset=1');
  }
  // replace the & characters with &
  // this is kinda hackish but not sure how to do it right
  $url = str_replace('&', '&', $url);

// If we are in a json context, respond appropriately
if (CRM_Utils_Array::value('snippet', $_GET) === 'json') {
CRM_Core_Page_AJAX::returnJsonResponse(array(
'status' => 'redirect',
'userContext' => $url,
));
}
```
self::setHttpHeader('Location', $url);
self::civiExit();
}