Add option to bundle the summary output

Having a filerev config like this:
```js
filerev: {
  options: {
    bundleSummary: true
  },
  images: {
    src: ['img1.png', 'img2.png'],
    dest: 'tmp'
  }
}
```

would result in a `grunt.filerev.summary` like this:

```js
{
  'images': {
    'img1.png': 'tmp/img1.59bcc3ad.png',
    'img2.png': 'tmp/img2.060b1aa6.png'
  }
}
```

#22