add example

cmd/example/main.go

@@ -0,0 +1,20 @@
+package main
+
+import "github.com/d5/tengo/script"
+
+var code = `
+reduce := func(seq, fn) {
+	s := 0
+    for x in seq { fn(x, s) }
+	return s
+}
+
+print(reduce([1, 2, 3], func(x, s) { s += x }))
+`
+
+func main() {
+	s := script.New([]byte(code))
+	if _, err := s.Run(); err != nil {
+		panic(err)
+	}
+}