change keyword names to valid Python identifiers

This commit mostly replaces all minus-signs ('-') in keyword names with
underscores ('_') to allow them to be used in filter-expressions. For
example 'gallery-id' got renamed to 'gallery_id'.

(It is theoretically possible to access any variable, regardless of its
name, with 'locals()["NAME"]', but that seems a bit too convoluted if
just 'NAME' could be enough)

gallery_dl/extractor/hentaicdn.py

@@ -16,8 +16,8 @@ import json
 class HentaicdnChapterExtractor(Extractor):
     """Base class for extractors for a single manga chapter"""
     subcategory = "chapter"
-    directory_fmt = ["{category}", "{manga-id} {title}"]
-    filename_fmt = ("{category}_{manga-id}_{chapter:>02}_"
+    directory_fmt = ["{category}", "{manga_id} {title}"]
+    filename_fmt = ("{category}_{manga_id}_{chapter:>02}_"
                     "{num:>03}.{extension}")
     url = ""