added mystery-box example macro (#787)

examples/macros/mystery-box.ts

@@ -0,0 +1,13 @@
+export function mysteryBox(callExpression) {
+  console.log(callExpression.log);
+  // get arguments
+  const [ countNode ] = callExpression.arguments;
+  const countString: string = countNode.get();
+  const count: number = parseInt(countString, 10);
+
+  // validate
+  if(!(count >= 1 && count <= 1000)) return new Error(`Argument ${countString} is expected to be between 1 and 1000`);
+
+  // return a value
+  return (Math.random() * count)|0;
+}