allow sorting negative numbers correctly

src/sigal/gallery.py

@@ -530,11 +530,11 @@ class Album:
             elif medias_sort_attr.startswith("meta."):
                 meta_key = medias_sort_attr.split(".", 1)[1]
                 key = natsort_keygen(
-                    key=lambda s: s.meta.get(meta_key, [""])[0], alg=ns.LOCALE
+                    key=lambda s: s.meta.get(meta_key, [""])[0], alg=ns.SIGNED|ns.LOCALE
                 )
             else:
                 key = natsort_keygen(
-                    key=lambda s: getattr(s, medias_sort_attr), alg=ns.LOCALE
+                    key=lambda s: getattr(s, medias_sort_attr), alg=ns.SIGNED|ns.LOCALE
                 )
 
             self.medias.sort(key=key, reverse=self.settings["medias_sort_reverse"])

tests/sample/pictures/dir1/test2/21.md

@@ -1,4 +1,4 @@
 Title: in"title"
-Order: 01
+Order: -10
 
 in"description