`looksSame` throws an error when an error is present using the readme code

`looksSame` throws an error when an error is present, using the readme code
```
looksSame('image1.png', 'image2.png', function(error, {equal}) {
// equal will be true, if images looks the same
});

> TypeError: Cannot destructure property `equal` of 'undefined' or 'null'.
```
Should probably update the docs to set the default in the callback i.e.
```
looksSame('image1.png', 'image2.png', function(error, {equal} = {}) {
// equal will be true, if images looks the same
});
```