Feature #457

- Tasks may now be given an 'until' date, after which they expire and are
  deleted.

src/main.h

@@ -79,6 +79,7 @@ void feedback_special_tags (const Task&, const std::string&);
 void feedback_unblocked (const Task&);
 std::string onProjectChange (Task&, bool scope = true);
 std::string onProjectChange (Task&, Task&);
+std::string onExpiration (Task&);
 
 // sort.cpp
 void sort_tasks (std::vector <Task>&, std::vector <int>&, const std::string&);