Bookmark support

[joogoo]

Please implement a simple bookmark system, it would be great to keep tracking of the progress of readings.

[keithstellyes]

This would be interesting, yeah.

Perhaps we could go for a simple JSON-based, maybe right by the ebook file /path/to/ebook/book.epub we have a /path/to/ebook/.bookmark.book.epub

I would prefer YAML even more since it's generally a prettier/easier-on-the-end-user format, I'm curious what others think