[danbooru] add option for extended metadata extraction (#1458)

* [danbooru] add option for extended metadata extraction

* appease linter

* [danbooru] update docs/configuration.rst

* [danbooru] rename extended-metadata -> metadata

gallery_dl/extractor/danbooru.py

@@ -32,6 +32,7 @@ class DanbooruExtractor(Extractor):
         super().__init__(match)
         self.root = "https://{}.donmai.us".format(match.group(1))
         self.ugoira = self.config("ugoira", False)
+        self.extended_metadata = self.config("metadata", False)
 
         username, api_key = self._get_auth_info()
         if username:
@@ -64,6 +65,14 @@ class DanbooruExtractor(Extractor):
                     url = post["large_file_url"]
                     post["extension"] = "webm"
 
+            if self.extended_metadata:
+                template = (
+                    "{}/posts/{}.json"
+                    "?only=artist_commentary,children,notes,parent"
+                )
+                resp = self.request(template.format(self.root, post["id"]))
+                post.update(resp.json())
+
             post.update(data)
             yield Message.Directory, post
             yield Message.Url, url, post