Create a new instance of `Iterator`

```js
var iterator = new Iterator();
```
@api public
function Iterator(options, fn) {
  if (typeof options === 'function') {
    fn = options;
    options = {};
  }
  if (typeof fn !== 'function') {
    throw new TypeError('Iterator expects `fn` to be a function.');
  }
  this.options = options || {};
  this.fn = fn;
}