I'd feel more comfortable if you did something like this:
```javascript
opt = opt || {};
opt.relativeTo = opt.relativeTo || path.resolve(path.dirname(file.path));
```

just to make sure you don't try writing the property `relativeTo` to an undefined variable (`opt`). I know its an edge case, but it's not inconceivable that someone could use this plugin without providing an `opt` object.