Add option to use white icons with KnpMenu

I'm using inversed bootstrap menu, so i need to use white icons, but there is no option to use it with menu.

Now you can add icon ot menu using this code:
```php
$menu->addChild('.icon-home Home', array('uri' => '#'));
```

i added option to select color:

```php
$menu->addChild('.icon-home(white) Home', array('uri' => '#'));
```