Disable auto archive if you leave the archive_filename option blank.

(Note that commenting/omitting archive_filename will still by default
write the archived entries to done.txt).

This addresses issue #42.

topydo/cli/CLIApplicationBase.py

@@ -203,7 +203,10 @@ class CLIApplicationBase(object):
         completed. It will do some maintenance and write out the final result
         to the todo.txt file.
         """
-        if self.todolist.is_dirty():
+
+        # do not archive when the value of the filename is an empty string
+        # (i.e. explicitly left empty in the configuration
+        if self.todolist.is_dirty() and config().archive():
             self._archive()
 
             if config().keep_sorted():