Automatically matches up template strings by name to output handlers. A handler can either be a string, which represents a method name of the helper, or it can be a closure or callable object. A handler takes 3 parameters: the value to be filtered, the name of the helper method that triggered the handler, and the array of options passed to the `_render()`. These handlers are shared among all helper objects, and are automatically triggered whenever a helper method renders a template string (using `_render()`) and a key which is to be embedded in the template string matches an array key of a corresponding handler.