Support FB2 format

[XVilka]

FictionBook is an open XML-based e-book format that originated and gained popularity in Russia. FictionBook files have the .fb2 filename extension. Some readers also support ZIP-compressed FictionBook files (.fb2.zip or .fbz).

See the specifications here: http://gribuser.ru/xml/fictionbook/index.html.en

[wasamasa]

This is kind of hard to do without performing what essentially amounts to a full rewrite, mostly because the code so far exploits some EPUB-specific things. It might be a good excuse to learn how generic functions work though.