Add ePub format output

[andrey-p]

This will most likely require studying the ePub format spec. Also finding what CSS is allowed by ePubs, and whether there's any difference between ePub vendors.

It might not be possible to do an automatic ePub conversion of all-image pages (a la http://www.apocalism.co.uk/read/naphthalene/#24) so maybe that should be left to the short story creator's discretion.

This might mean that extra care needs to be put for what styling hooks are being offered.