Added mrb_str_index to the mrb API

include/mruby/string.h

@@ -84,6 +84,12 @@ MRB_API mrb_int mrb_str_strlen(mrb_state*, struct RString*);
 
 void mrb_gc_free_str(mrb_state*, struct RString*);
 MRB_API void mrb_str_modify(mrb_state*, struct RString*);
+
+/*
+ * Finds the index of a substring in a string
+ */
+MRB_API mrb_int mrb_str_index(mrb_state*, mrb_value, const char*, mrb_int, mrb_int);
+
 /*
  * Appends self to other. Returns self as a concatnated string.
  *