Bug - Durations

- Duration were parsed as <int><unit> in A3, whereas <number><unit> is
  correct.
- Added Nibbler::str method for internal access.
- Nibbler::save, ::restore now return positions.  Useful for extracting
  substrings.
- Modified countdown.t to use the (now) correct units for durations.

src/Nibbler.h

@@ -80,9 +80,9 @@ public:
   std::string next (const int quantity);
 
   std::string::size_type cursor ();
-
-  void save ();
-  void restore ();
+  std::string::size_type save ();
+  std::string::size_type restore ();
+  const std::string& str () const;
 
   bool depleted ();