two bug fixes

* use last_xpointer to keep reading progress for crereader

adapted from commit https://github.com/houqp/kindlepdfviewer/commit/e7e6a2be9c3258c452a8329db3c2d56638e99480

This fix a bug in crereader, i.e. when in page mode, keep reopening a document gives you different pages each time.

* reset self.toc on document reformat as discussed in #692