Add xhrCallback callback func to opts and call it on success request

You may need to do some extra stuff with result XMLHttpRequest
when it's done. So here is a callback function that will be called
everytime when new page's request was finished successfuly.

In my case I need this to get some data (pagination cursor for next
page) from extra HTTP header each time when a content loaded from a
destination resource.
Merge branch 'xhr_callback' into all2cart