add a class if character limit exceed's 

add a class if the character count exceeds the limit. 
For example, 
If we want to change the color of couter_label to red when limit exceeds,

we can have a class
```css
.limit{
   color:red
}
```

we call the counter function with options 
```javascript
$('textarea[data-max-length]').counter({'limitExceedClass':'limit'});
```