#5 Easy function to fetch the path/filename of the cover image (if any)

[Mateusz Viste]

Fetching the cover file of an epub file is a quite widespread need (I suppose), but libepub do not provide an easy way to do it. Currently one would have to parse manually the opf list and look for the cover.

Would be really nice to have some easy-to-use function like get_cover_filename(), that would provide the filename of the cover file inside the ziped epub archive (or NULL, if no cover is present).