Support Source Maps like grunt-autoprefixer

https://github.com/nDmitry/grunt-autoprefixer#optionsmap

```javascript
css_mqpacker: {
  main: {
    options: {
      // `dest/css/output.css.map`
      map: true
    },
    src: 'src/css/input.css',
    dest: 'dest/css/output.css'
  }
}
```