Provide hook to override thumbnail image URL

Provide a hook to allow modules to override the item's thumbnail as provided by CONTENTdm. This hook should reside in theme_contentdm_thumbnail(). While themers can override this function using the usual methods, this hook only alters the thumbnail's URL, depending on the item's alias or pointer (the only two item-specific parameters passed to theme_contentdm_thumbnail()), so, for example, you could implement the hook to provide a URL to an alternative thumbnail for an entire collection or a specific item.

The hook should take the attributes $alias, $pointer, $url, so implementors can provide the URL to the thumbnail they want to use.