Enable less/font-awesome.less to support Less mixin

Be more sementic and decouple font-awesome class name off from html code by Less class mixin.

```css
// before
<span class='icon-check'></span>

// after
<span class='icon-my-check-class-name'></span>
.icon-my-check-class-name{
  .icon-check;
}
```