rowClass

adding rowClass as a function or string. The rowClass as function will receive the current element. Very useful. I've wrote an example into README. Here's why and how I'm using:


```js
rowClass: function(item) {
  var qnt = item.qnt;
  //
  switch (qnt) {
    case 0:
      return 'danger';
    case 1:
    case 2:
      return 'warning';
    default:
      return ''
  }
},
```

as a string

```js
rowClass: 'danger',