Add `grunt update-image` task.

I added a `grunt update-image` task that will create a image of the current result of `compile` use [`node-atom-screenshot`](https://github.com/FWeinb/node-atom-screenshot) to take a screenshot and than use [`grunt-gh-pages`](https://github.com/tschaub/grunt-gh-pages) to push the preview image to the `gh-pages` branch of [SassDoc/sassdoc.github.io](https://github.com/)

So to update the image someone just has to run `grunt update-image`