Feature Request: Callbacks on page turn

[beville]

This is sort of a pull request, I guess, but I haven't forked yet.

In my ComicStreamer app where I'm using your reader code, I've added a callback for when the user turns the page. My particular use case is to set a bookmark on the server. You can see my mods here:

https://github.com/beville/ComicStreamer/blob/master/comicstreamerlib/static/comicbook/js/comicbook.js

Just look for "pageTurnCallback"