Clean up page template class in body tag

This might get shot down, and I'm completely open to discussion, but I wanted to see what the community thought. I will attempt to make my case cleanly below.

### Clean up page template class in Body tag
This PR updates the already present `roots_body_class` function to also clean up the class for page templates.
The default class added for a page template looks like `page-template-template-custom-php`. The patch cleans up the class to be `template-custom`.

#### Pros
The main reason I decided to do this revolves around cleaner code when using the DOM-Routing in javascript to target a specific page template. Instead of:

```javascript
    page_template_template_custom_php: {
        init: function() {
          // JavaScript to be fired on a page using this template
        }
    }
```
We can instead do: 

```javascript
    template_custom: {
        init: function() {
          // JavaScript to be fired on a page using this template
        }
    }
```
#### Cons
I haven't tested this extensively against any other plugins, and I am also not sure if any other plugins may depend on the original class. I've never run into one, but I feel it's worth mentioning. 

This change would also possible be breaking to those who attempt to update an already existing theme to a newer version of Roots if they have any routing associated to a template using the old class. 

###########

Thoughts?