Support for prefix-option, allowing empty if required

I'm trying to achieve this with a custom mustache-template:

```mustache
icon,
i[class^='icon-'],
i[class*=' icon-'] {
  display: inline-block;
  background-image: url({{&std.0.sprite}});
}

{{#std}}
icon.{{&name}},
icon[name='{{&name}}'],
i.icon-{{&name}} {
  width: {{width}}px;
  height: {{height}}px;
  background-position: -{{x}}px -{{y}}px;
}
{{/std}}
```

So, I've added a some images i.e. `sprites/email.png` and now can I use them as follows:

```html
<icon name="email">
<i class="icon-email"></i>
```

Of course, to achieve this is required an empty-prefix value.

What do you think?