Emit an event when a post/page already exists, also.

Creation of new_post successfully emits an event, but no event is fired
new_post fails because there is an existing post.  This event can be
used by plugins, for e.g. open the post in an editor or prompt to delete
the post, etc.

nikola/plugins/command/new_post.py

@@ -349,6 +349,13 @@ def _execute(self, options, args):
 
         if (not onefile and os.path.isfile(meta_path)) or \
                 os.path.isfile(txt_path):
+
+            # Emit an event when a post exists
+            event = dict(path=txt_path)
+            if not onefile:  # write metadata file
+                event['meta_path'] = meta_path
+            signal('existing_' + content_type).send(self, **event)
+
             LOGGER.error("The title already exists!")
             exit(8)