Content field for slider

Sometimes slider's handle should contain custom content, e.g. icon or some text. For instance, audio player volume control:

![](http://yadi.sk/d/zIGXPEAISME8f_XXL.jpg)

I've added `content` field similar to other controls.

```
nb-slider({
    'id': 's3'
    'size': 's'
    'content': apply / my-slider-content-handle
})

match / my-slider-content-handle {
    nb-icon({
         'icon': 'volume'
    })
}
```