Bookmarks from ComicRack's ComicInfo.xml are now converted to EPUB
chapters. `Bookmark` is an attribute to the `Page` element in
ComicInfo.xml.
* Works with flat directory structure (no sub-folders support)
Directories and files returned by walk() are not always sorted.
Currently we sort the filelist after creating the it. When walk()-ing
topdown (we do) the directory list can be modified in-place. By sorting
the directories and files before creating the filelist, we guarantee
a sorted chapter list.
The walk() function does not necessarily return filenames in
alphabetical order. This leads to a random cover image.
Sorting filenames before setting the cover images fixes this issue.