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/CmdTimesheet.cpp

@@ -50,6 +50,7 @@ int CmdTimesheet::execute (std::string& output)
 {
   int rc = 0;
 
+/*
   // Scan the pending tasks.
   std::vector <Task> tasks;
   context.tdb.lock (context.config.getBoolean ("locking"));
@@ -179,6 +180,7 @@ int CmdTimesheet::execute (std::string& output)
   }
 
   output = out.str ();
+*/
   return rc;
 }