Add roots_body_class function to check for page slug in body_class and add it as needed.

This PR adds a function `roots_body_class` in `utils.php` that checks for the page slug in body_class and adds it as needed. If it is present already, it skips over. This addresses issue: https://github.com/roots/roots/issues/1164
can you replace the tabs with 2 spaces please to match the rest of the project?