Use callback functions over options.then

[whizkydee]

The API as at v1.1.0 provides options.then to be able access input and result.

reverse('dog', {
  then: (a, b) => `${a} was changed to ${b}`
}) 

From v2.0.0 however, we should use callback functions as the third argument over options.then.

options would take in an object or falsy values which would implicitly get converted to an empty object.

reverse('dog', null, (a, b) => `${a} was changed to ${b}`)