Classify will provide the ability to add classes (and other attributes) on any entity type.
Allow a user configurable field to allow adding custom classes/attributes to an entity.
Allow a predefined set of classes/attributes to be provided by a module/theme, allowing a user to select a class/attribute that already has 'functionality' attached to it.
Each type of attribute (class, id, title, data-x, etc.) should be setup via a Plugin.