implement (sub)category-transfer between extractors (#41)

ImageFap- and all Manga-Extractors will transfer their (sub)category
values to other extractors instantiated by them, which will in turn
allow those to use options set for their parents.

Example:
ImagefapGalleryExtractors will use options set under
extractor.imagefap.user, if (and only if) they have been instantiated by
a ImagefapUserExtractor; and options from extractor.imagefap.gallery
otherwise.

gallery_dl/extractor/common.py

@@ -25,6 +25,7 @@ class Extractor():
 
     category = ""
     subcategory = ""
+    categorytransfer = False
     directory_fmt = ["{category}"]
     filename_fmt = "{filename}"
     cookiedomain = ""
@@ -147,6 +148,7 @@ class AsynchronousExtractor(Extractor):
 class MangaExtractor(Extractor):
 
     subcategory = "manga"
+    categorytransfer = True
     scheme = "http"
     root = ""
     reverse = True