Adds support for sourcemaps option in the less task.

```js
roto.addTask('less', {
    compress : false,
    sourcemaps: true,
    files    : 'public/css/www/stylesheet.less',
    output   : 'public/css/www/stylesheet.css'
});
```

http://robdodson.me/blog/2012/12/28/debug-less-with-chrome-developer-tools/