merge 'bypost' functionality into metadata postprocessor

gallery_dl/postprocessor/common.py

@@ -26,6 +26,10 @@ class PostProcessor():
     def run(pathfmt):
         """Execute the postprocessor for a file"""
 
+    @staticmethod
+    def run_metadata(pathfmt):
+        """Execute the postprocessor for a file"""
+
     @staticmethod
     def run_after(pathfmt):
         """Execute postprocessor after moving a file to its target location"""