add extractors for gelbooru v0.1 sites

- support https://illusioncards.booru.org/  (closes #426)
- support https://the-collection.booru.org/ (closes #767)
- support https://allgirl.booru.org/
- closes #234, closes #473, closes #1238

To get gallery-dl to recognize other sites running Gelbooru v0.1
(most sites on booru.org), add one or more entries to the
'gelbooru_v01' block in your config file. For example:

{
    "extractor": {
        "gelbooru_v01": {
            "rozenmaidenbooru": {"root": "http://rm.booru.org"},
            "drawfriendsbooru": {"root": "http://drawfriends.booru.org"}
        }
    }
}

gallery_dl/extractor/__init__.py

@@ -35,6 +35,7 @@ modules = [
     "furaffinity",
     "fuskator",
     "gelbooru",
+    "gelbooru_v01",
     "gelbooru_v02",
     "gfycat",
     "hbrowse",