[danbooru:pool] download posts in pool order (#7091)

- add 'order-posts' option
- add 'num' metadata field for pool position
- update default filenames to order by pool position

docs/gallery-dl.conf

@@ -769,7 +769,11 @@
             "external" : false,
             "metadata" : false,
             "threshold": "auto",
-            "ugoira"   : false
+            "ugoira"   : false,
+
+            "pool": {
+                "order-posts": "pool"
+            }
         },
         "danbooru":
         {