Expose document progress to hooks

[Jesse-Bakker]

A hook I would love to implement is progress synchronization across devices. At the moment, reading progress is however not exposed to hooks.

Currently, reading progress is internally converted to "pages", but for reflowable documents, such as epub, that is not ideal, as the page number calculation is specific to plato. Of course, a hook implementation could work around that, but I'd suggest exposing reading progress as both page number and optionally a format specific link, which would link to the last viewed element (such as a page for pdf, or epubcfi for epub).