epub parser: separate text blocks of logical elements by "Form Feed"

[workflowsguy]

When combining the text read from individual book elements of an epub file, those elements are currently separated only by an '\n' character.

I suggest separating them by a '\f' character instead. This would be analogous to current text extraction from PDF files, where the "logical elements" "individual pages" are also separated by a Form Feed.

This would help to maintain at least some kind of structure of the original file in the resulting txt file and thus make parsing the logical structure possible.