allow default title shown on the menu dropdown

Example:  **default-label="All Genres"**

```
<div 
    multi-select 
    input-model="$scope.arrOfObjects"
    output-model="$scope.arrOfObjects2"
    button-label="property1 property2 ..."         
    item-label="property1 property2 ..." 
    tick-property="property3" 
    disable-property="property4"
    orientation="horizontal | vertical"
    selection-mode="multiple | single"
    max-labels="999"     
    directive-id="..."
    is-disabled="true | false"
    helper-elements="all none reset filter"
    on-open="$scope.functionOpen()"
    on-close="$scope.functionClose()"
    on-focus="$scope.functionFocus()"
    on-blur="$scope.functionBlur()"   
    default-label="default text"
>
</div>
```