Filters

- Implemented Command::filter_shortcut to detect when a filter begins
  with "status:pending" and skip the loading/parsing of completed.data.
- Removed obsolete att.t.cpp tests.
- Removed diagnostics from TDB2::TDB2 that were causing a segfault.
  I suppose there is no std::cout available during global ctors?

src/commands/Command.h

@@ -55,6 +55,7 @@ public:
 protected:
   void filter (std::vector <Task>&, std::vector <Task>&);
   void filter (std::vector <Task>&);
+  bool filter_shortcut (const Arguments&);
 
   void modify_task_description_replace (Task&, Arguments&);
   void modify_task_description_prepend (Task&, Arguments&);