Add duration, delay, iteration and offset support

Add the ability to customize the duration, delay, iteration and offset via a JavaScript (global config) and via HTML case by case.

- HTML example

```html
<div class="wow" data-duration="2s" data-delay="2s" data-iteration="2" data-offset="10"></div>
```

```javascript
new WOW(
  {
    offset:       100,        // default
    duration:     '1s',       // default
    delay:        '0s',       // default
    iteration:    '1'         // default
  }
).init();
```

This adds so much more potential to the library!

Feedback welcome /cc @attilaolah