add .center class

When using ruban, I find myself willing to center element a lot of time.

It would be useful to add a .center class to center element on a slide.

```
<section>
    <h1>A centered title</h1>
    <p>a centered paragraphe</p>
</section>
```

Best regards
add .center class
Merge pull request #10 from lauterry/master

add .center class