Allow "Page source" link to be hidden

Docs hosted on ReadTheDocs already give you  the option to view and edit a page's source on GitHub. 

![quickstart_ _marshmallow_1_1_0_documentation](https://cloud.githubusercontent.com/assets/2379650/5329112/84179efa-7d6c-11e4-87a7-e1cf664ea649.png)

This makes the "Page source" link redundant. This patch allows a user to hide the link.