Add clean-css options

Example of new configuration:

```
grunt.initConfig({
    stripmq: {
        //Viewport options
        options: {
            width: 1000,
            type: 'screen',
            cleanCss: {
                compatibility: 'ie8',
                keepBreaks: true,
                noAdvanced: true
            }
        },
        all: {
            files: {
                //follows the pattern 'destination': ['source']
                'css/app-old-ie.css': ['css/app.css']
            }
        }
    }
});
```