Adding optional define ISAAC_IDX_TYPE for changing the indextype using by isaac

In the future this should probably some kind of template parameter instead
of another macro, but otherwise I can't keep types like "isaac_size2", so
changing this would be a bigger, backwards compatibility breaking change

Author: theZiz

example/example.cpp

@@ -13,6 +13,7 @@
  * You should have received a copy of the GNU General Lesser Public
  * License along with ISAAC.  If not, see <www.gnu.org/licenses/>. */
 
+#define ISAAC_IDX_TYPE int
 #include <isaac.hpp>
 
 #include "example_details.hpp"