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/mangapanda.py

@@ -35,5 +35,5 @@ class MangapandaChapterExtractor(MangapandaBase, MangareaderChapterExtractor):
     ]
     test = [("http://www.mangapanda.com/red-storm/2", {
         "url": "4bf4ddf6c50105ec8a37675495ab80c46608275d",
-        "keyword": "89c712f7ed255ec9c1d8e84dcb5a160b6cb4498c",
+        "keyword": "16c881a09f5cf0f2c1c374665b667d1f26b20c22",
     })]