Events need a way to chain string data on a call.

[adampatterson]

For example, If I want to build up a stack of classes for body_class() I don't want to create a bunch of senseless methods to pass class names on.

I should be able to append to a list, remove an item from the list, and return the list.

on return optionally offer string 'one two three' or array().

Relates to #25