handler = function () {
            callback.call(e, count(e, options))
          }
The `on` method binds the counting handler to all given elements. The
event is either `oninput` or `onkeydown`, based on the capabilities of
the browser.

@param   {Nodes}     elements   All elements that should receive the
                                Countable functionality.

@param   {Function}  callback   The callback to fire whenever the
                                element's value changes. The callback is
                                called with the relevant element bound
                                to `this` and the counted values as the
                                single parameter.

@param   {Object}    [options]  An object to modify Countable's
                                behaviour.

@return  {Object}    Returns the Countable object to allow for chaining.