Change : packageSpecific.files option behavior

Hi,

I suggest an improvement to your great tool. While trying to deploy jquery.min.js in my production enviroment in thought it would make sense to not copy any other files than the ones explicitly stated in packageSpecific.files option.

For example with the config:
```js
packageSpecific: {
    'jquery': {
        files: [
            'dist/jquery.min.js'
        ]
    }
},
```

Before :
**jquery.js** and **jquery.min.js** are deployed into dest/.

After :
only **jquery.min.js** is deployed into dest/.