Added the ability to store the last URL hash fragment in the exported notebook. This is useful as a lightweight, user editable persistence mechanism. Internally the state is saved as a fileattachment to survive export, so no changes to the file format were required. I needed this to provide a "default" viewof the notebook graph. hash URL is limited in size, but you can use the mutable fileattachments for big binary data. I just think the hash URL is somewhat web idiomatic for certain things and its also user editable so its nice to surface (and then persist) for certain types of things.