First attempt at data generator support

The support is to be considered experimental, that is, the interfaces,
the first party generators and helper functions can change or be removed
at any point in time.

Related to #850

include/catch.hpp

@@ -62,6 +62,7 @@
 #ifndef CATCH_CONFIG_DISABLE_MATCHERS
 #include "internal/catch_capture_matchers.h"
 #endif
+#include "internal/catch_generators.hpp"
 
 // These files are included here so the single_include script doesn't put them
 // in the conditionally compiled sections