Add more Infinite Scroll jQuery events

Following request @georgestephanis in issue #1488, I'm opening this ticket to request the `post-load` event in the Infinite Scroll jQuery plugin to be namespaced. It would be useful to add also some more extension points, for example a `pre-load` event or something that allows the editing of the `query_args` from client side before sending the AJAX request. As event data it would be useful to have access to the `Scroller` instance, to edit it's internal variables if needed (like the page number).

Start with the Infinite Scroll jQuery plugin and review the request in issue #1488 for context. The requested scope includes namespacing post-load, possible pre-load or query_args hooks, and event access to the Scroller instance; done requires agreeing on and implementing a coherent extension-point API.

Written by the indexing model from the issue text.