Add support for setting a value for "prompt" option.

I'd like to propose adding in this change that will add support for setting a value for the "prompt" option.

Example: 
```php
$form->field($model, 'campaign_id')->dropDownList($campaigns, ['prompt' => [0 => 'All']]);
```

Main reason is because either MySQL or Yii recently changed and i'm getting a lot of MySQL errors regarding trying to insert NULL values (when really I just want to insert 0)