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/CmdModify.cpp

@@ -51,6 +51,7 @@ CmdModify::CmdModify ()
 int CmdModify::execute (std::string& output)
 {
   int count = 0;
+/*
   std::stringstream out;
 
   std::vector <Task> tasks;
@@ -181,6 +182,7 @@ int CmdModify::execute (std::string& output)
     out << "Modified " << count << " task" << (count == 1 ? ".\n" : "s.\n");
 
   output = out.str ();
+*/
   return 0;
 }