Toggles the existence of a class on an element. If the class exists it will be removed, if it doesn't it will be added.
If a condition argument is supplied, the truthiness of that condition is what determines whether the class should exist on the element or not. This simplifies common use case of an element needing a class if condition is true. condition can be passed as either a primitive which will eventually coerce to boolean or as a function. If you pass a function, you get current element passed as a first argument.