[artstation] add 'date' metadata field (#839)

gallery_dl/extractor/artstation.py

@@ -66,6 +66,8 @@ class ArtstationExtractor(Extractor):
         data["title"] = text.unescape(data["title"])
         data["description"] = text.unescape(text.remove_html(
             data["description"]))
+        data["date"] = text.parse_datetime(
+            data["created_at"], "%Y-%m-%dT%H:%M:%S.%f%z")
 
         assets = data["assets"]
         del data["assets"]