support writing output to multiple files, with different JSON data

This PR allows the user to write the JSON test data to multiple files based on the same schema.
E.g.:
```javascript
faker: {
	slideDeck: {
		options: {
			jsonFormat: '<%= yeoman.app %>/data/slideDeck.faker.v1.js',
			out: ['<%= yeoman.app %>/data/slideDeck1.json', '<%= yeoman.app %>/data/slideDeck2.json']
		}
	}
}
```