End temporary file names with .todo.txt

That way the filetype detection of the todo.txt vim plugin is triggered and shows the appropriate syntax highlighting.

https://github.com/freitass/todo.txt-vim

topydo/commands/EditCommand.py

@@ -55,7 +55,7 @@ class EditCommand(MultiCommand):
             self.multi_mode = False
 
     def _todos_to_temp(self):
-        f = tempfile.NamedTemporaryFile(delete=False)
+        f = tempfile.NamedTemporaryFile(delete=False, suffix='.todo.txt')
         for todo in self.todos:
             f.write((todo.source() + "\n").encode('utf-8'))
         f.close()