Allow specifying unpacked directory location

I am packaging an application in Electron and using asar to do this. My application has a `cfg` folder that end users are allowed to edit to change core behavior of my application. I've specified all files in the `cfg` folder to remain unpacked, but this means users would navigate to `resources/app.asar.unpacked/cfg` instead of just `cfg` from the root level of the installation to make their changes.

It would be nice if there was a means by which to specify where unpacked files for asar are stored. Something like the following, maybe:

```
asar.createPackageWithOptions(appDir, buildDir, {"unpack": "*/cfg/*", "unpackedDirectory": "../"}, function () {
...
});
```