[booru] add an option to extract notes (only gelbooru for now) (#1457)

* [booru] add an option to extract notes (currently implemented only for gelbooru)

* appease linter

* [gelbooru] rename "text" to "body" in note extraction

* add a code comment about reusing return value of _extended_tags

gallery_dl/extractor/gelbooru.py

@@ -108,4 +108,26 @@ class GelbooruPostExtractor(GelbooruBase,
             "pattern": r"https://img\d\.gelbooru\.com/images"
                        r"/22/61/226111273615049235b001b381707bd0\.webm",
         }),
+        # notes
+        ("https://gelbooru.com/index.php?page=post&s=view&id=5997331", {
+            "options": (("notes", True),),
+            "keywords": {
+                "notes": [
+                    {
+                        "height": 553,
+                        "body": "Look over this way when you talk~",
+                        "width": 246,
+                        "x": 35,
+                        "y": 72
+                    },
+                    {
+                        "height": 557,
+                        "body": "Hey~\nAre you listening~?",
+                        "width": 246,
+                        "x": 1233,
+                        "y": 109
+                    }
+                ]
+            }
+        }),
     )