Added support for omitting data-step

You may now add some elements with data-steps and some without.. The feature is that they elements are ordered as you expect. 

### Before
``` html
<div data-intro="This is shown first"></div>
<div data-intro="second..."></div>
<div data-intro="This is shown last" data-step="1"></div>
``` 

### With this feature
``` html
<div data-intro="second..."></div>
<div data-intro="This is shown last"></div>
<div data-intro="This is shown first" data-step="1"></div>
```