Feature request: add ability to prevent modifying certain value in `unflatten`

I have a use case where I'd like to prevent the `target`'s value from being modified in `unflatten`. There's probably a number of ways to approach it but here's what I was thinking:

```js
// prevent modifying any object with a value of 'testValue'
unflatten(obj, {filter: {value: 2}})

// prevent modifying any object with a key of 'thing'
unflatten(obj, {filter: {key: 'thing'}})

// prevent modifying the object {thing: 2}
unflatten(obj, {filter: {key: 'thing', value: 2}})
```