Allow multiple files to be processed using files: object model

I simply moved your main logic into the loop, so we could process multiple files using something like:

```js
grunt.initConfig({
  'less-imports': {
    dev: {
      files: {
        'dest/vendor.less': ['src/vendor/a.less', 'src/vendor/b.less'],
        'dest/widgets.less': ['src/widgets/buttons.less']
      },
    }
});
```

Not sure why you hardcoded files[0]. It seems your tests are passing and my app now generates the files properly.