Argument Parsing

- Eliminated stored arg_overrides and file_override in Context.
- Removed Filter, Subst, Task, Sequence from Context.
- Remove shadow file support.  Hallelujah.
- Disabled/commented out most commands, ready for the big transition
  to the new parsing style.
- Obsoleted Subst.{h,cpp}.

src/commands/CmdUrgency.cpp

@@ -46,6 +46,7 @@ CmdUrgency::CmdUrgency ()
 ////////////////////////////////////////////////////////////////////////////////
 int CmdUrgency::execute (std::string& output)
 {
+/*
   // Get all the tasks.
   std::vector <Task> tasks;
   context.tdb.lock (context.config.getBoolean ("locking"));
@@ -73,6 +74,7 @@ int CmdUrgency::execute (std::string& output)
         << "\n";
 
   output = out.str ();
+*/
   return 0;
 }