Extracts EPUB metadata via the bupe
package. Pure Elixir, no system dependency.
Returned fields (only those present in the EPUB are included):
:title, :creator, :contributor, :publisher, :language,
:identifier, :date, :description, :subject, :rights,
:source, :pages.
:pages is the number of items in the EPUB spine (chapter count),
not a word/page count.