Send a plug-in event when file is moved

I use this in one of my plug-ins to notice when I've moved all the audio
files in an album from one directory to another, at which point I move
any associated non-album files to the new directory and delete the old
directory.

beets/library.py

@@ -370,6 +370,7 @@ class Item(object):
             util.copy(self.path, dest)
         else:
             util.move(self.path, dest)
+            plugins.send("item_moved", source=self.path, destination=dest)
 
         # Either copying or moving succeeded, so update the stored path.
         self.path = dest