Add page template class to body tag

This PR stems from a discussion around a [similar proposed PR](https://github.com/roots/roots/pull/1229) on roots.

This PR enhances the `soil_body_class` function to add a class based on the page template used. This allows for cleaner CSS and, if using roots, a cleaner selector when implementing template specific Javascript in the DOM-Routing methods used by Roots. 

Please refer to the above mentioned Closed PR for a more in-depth discussion as to the merits of this PR.