Add a plugin hook to specify a class on the body of the document

lib/plugin/instance.rb

@@ -103,6 +103,12 @@ class Plugin::Instance
     end
   end
 
+  def add_body_class(class_name)
+    reloadable_patch do |plugin|
+      ::ApplicationHelper.extra_body_classes << class_name if plugin.enabled?
+    end
+  end
+
   # Extend a class but check that the plugin is enabled
   # for class methods use `add_class_method`
   def add_to_class(class_name, attr, &block)