Add option opens: 'center'

In addition to 'left' and 'right', now 'center' can be used for the 'opens' option.
The result looks like this:

![screen shot 2014-10-16 at 3 06 23 pm](https://cloud.githubusercontent.com/assets/150845/4662776/478be61c-5535-11e4-8f62-6277cf846e65.png)