queryString option

Hi,

I've added a queryString option to your script to fit my needs. I might interests you.

This new option allow you to add the hash as a querystring at the end of the file references instead of renaming the file, allowing you to use it as a cache buster.

Usage :

```js
version: {
  queryString: true,
  myapp: {
    phases: [
      {
        files: [
          'images/*.png'
        ],
        references: [
          'css/*.css'
        ]
      }
    ],
    version: 'build/version.json'
  }
}
```

For example, all the "image/cats.png" in the css files will be renamed to "images/cats.png?61ab12ee". The original filename remains untouched.
Of course, the version.json is updated accordingly.

PS: I do not have updated the README.md, as the main task of the plugin is to "Rename files" (which is no longer so true with this option) this plugin might require a new description.