Support bookmarks

[lentzi90]

I think bookmark support should be added. Just saving the current place is usually enough but sometimes custom bookmarks would be nice!

[rschroll]

I could see this to be useful with reference texts. Presumably we'd
want to have the ability to give labels to the bookmarks (especially
because page numbers don't really work with epubs). These could be
displayed as an alternative to the contents page. I'm not sure how
you'd switch between them, but I'm sure we could come up with something.