[pp:metadata] add '"mode": "print"' (#2691)

https://github.com/mikf/gallery-dl/issues/2691#issuecomment-3276158980

gallery_dl/postprocessor/metadata.py

@@ -45,6 +45,15 @@ class MetadataPP(PostProcessor):
                 cfmt = "\n".join(cfmt) + "\n"
             self._content_fmt = formatter.parse(cfmt).format_map
             ext = "txt"
+        elif mode == "print":
+            nl = "\n"
+            if isinstance(cfmt, list):
+                cfmt = f"{nl.join(cfmt)}{nl}"
+            if cfmt[-1] != nl and (cfmt[0] != "\f" or cfmt[1] == "F"):
+                cfmt = f"{cfmt}{nl}"
+            self.write = self._write_custom
+            self._content_fmt = formatter.parse(cfmt).format_map
+            filename = "-"
         elif mode == "jsonl":
             self.write = self._write_json
             self._json_encode = self._make_encoder(options).encode