added:  'onSet' & 'onRemove' callback support

added:  'onSet' & 'onRemove' callback support
Hm..
```js
if( typeof options.onSet === 'undefined' ) {
} else {
  if( typeof options.onSet === 'function' ) {
    ..
  }
}
```

This can just be reduced to:

```js
if( typeof options.onSet === 'function' ) {
  ..
}
```

Or even:

```js
if( options.onSet ) {
  ..
}
```