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/CmdCount.cpp

@@ -45,6 +45,7 @@ CmdCount::CmdCount ()
 ////////////////////////////////////////////////////////////////////////////////
 int CmdCount::execute (std::string& output)
 {
+/*
   // Scan the pending tasks, applying any filter.
   std::vector <Task> tasks;
   context.tdb.lock (context.config.getBoolean ("locking"));
@@ -63,6 +64,7 @@ int CmdCount::execute (std::string& output)
   std::stringstream out;
   out << count << "\n";
   output = out.str ();
+*/
   return 0;
 }