Add guess a number example.

example/guess_number.wren

@@ -0,0 +1,23 @@
+// The program picks a random number from 1 to 100 and you have to guess it.
+// Each time you guess, it tells you if your guess is too low, too high, or on
+// the money.
+
+import "io" for Stdin
+import "random" for Random
+
+var number = Random.new().int(100) + 1
+
+while (true) {
+  System.write("Guess a number: ")
+  var guess = Num.fromString(Stdin.readLine())
+  if (guess == null) {
+    System.print("That's not a number!")
+  } else if (guess < number) {
+    System.print("Too low.")
+  } else if (guess == number) {
+    System.print("You win!")
+    break
+  } else {
+    System.print("Too high.")
+  }
+}