Add the ability to use dashes as well as dots for fencing yaml

GitHub has some nice conventions for displaying yaml data, like this:
![](https://i.cloudup.com/WeXzjJ8rcG-3000x3000.png)
__but__ it only will do so for documents that have their yaml fenced by ``---``. Otherwise you just get:
![](https://i.cloudup.com/JASsEFIDc6-3000x3000.png)

This pull request makes it possible to use either ``...`` or ``---``, so no existing apps should be impacted.

Thanks! :)