support tumblrgallery.xyz (#1298)

* support `tumblrgallery.xyz`

* fix format issues

* Refactor and add post and search page support

* Fix warnings

* Few improvments

* Better file names

* Fix linting errors

* move id closer to the begining of the file name

Co-authored-by: topozorra <none>

gallery_dl/extractor/__init__.py

@@ -113,6 +113,7 @@ modules = [
     "subscribestar",
     "tsumino",
     "tumblr",
+    "tumblrgallery",
     "twitter",
     "unsplash",
     "vanillarock",