Add utility to index ePub documents content

Content of ePub documents is not indexed for full text search, while on some ZIM it is the "core" of the ZIM.

Extracting ePub info would be beneficial to multiple scrapers and should thus ideally be exposed in scraperlib, just like we did for PDFs in https://github.com/openzim/python-scraperlib/pull/182

See https://github.com/openzim/gutenberg/issues/95