'/?&#' -> '/?#' and '?&#' -> '?#'
According to https://www.ietf.org/rfc/rfc3986.txt, URLs are
"organized hierarchically" by using "the slash ("/"), question
mark ("?"), and number sign ("#") characters to delimit components"
- let the GalleryExtractor class inherit directly from Extractor
- make ChapterExtractor a subclass of GalleryExtractor
- change enumeration field names of GalleryExtractors to 'num'