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/Task.h

@@ -30,7 +30,6 @@
 
 #include <string>
 #include <Record.h>
-#include <Subst.h>
 #include <Sequence.h>
 
 class Task : public Record