Add roots_display_sidebar filter

Add filter to sidebar display to have the ability to change when it is displayed in child themes and other circumstances.

https://github.com/retlehs/roots/issues/767

lib/config.php

@@ -67,7 +67,7 @@ function roots_display_sidebar() {
     )
   );
 
-  return $sidebar_config->display;
+  return  apply_filters('roots_display_sidebar', $sidebar_config->display);
 }
 
 /**