Add custom "more" label callback

Now users could pass custom callback to generate "more" label. It's only solution when you don't want to use hardcoded "Total (x)" (e.g. you need to translate it (i18n)).

Template:

```html
<multi-select more-label="generateMoreLabel(items)"></multi-select>
```

Controller:

```javascript
$scope.generateMoreLabel = function (items) {
  return "There is " + items.length + " items";
};
```