add second argument for custom application name

RE: https://github.com/isimmons/gulp-notify-growl/tree/feature/allow-custom-application-name

Allows multiple growlNotifiers with custom application names to register

Example:

```
var growlPHPUnitNotifier = growl({}, 'PHPUnit Pass!');

var growlSassNotifier = growl({}, 'Sass Compiled!');
```

Or as I'm using with phpunit for pass and fail notifiers
```
var growlNotifierFail = growl({
    icon : fs.readFileSync('doge.png')
}, 'Very Fail!');

var growlNotifierSuccess = growl({
    icon : fs.readFileSync('doge.png')
}, 'Very Unit!');
```

Otherwise, my notifications have a big huge 'Gulp' at the top above my custom title. Plus this allows further customization of application settings in growl.