Add support for an include option (in addition to ignore)

### Preflight Checklist

* [x] I have read the [contribution documentation](https://github.com/electron-userland/electron-packager/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [code of conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project follows, as appropriate.
* [x] I have searched the issue tracker for a feature request that matches the one I want to file, without success.

### Problem Description

AFAICS (apologies if I missed smth) electron-packager has a way to ignore paths from getting copied to the output artifact, but blacklisting is a risky way to decide what goes in. Add a file with sensitive info to the repo, forget to add it to ignore, and BAM!

A whitelist is stable and does answer directly the question: what goes into the artifact?

### Proposed Solution

In the best of worlds, you should just follow `npm pack`, meaning respect the `files` section in `package.json`, and if none - respect the `.npmignore` (or just stick to the current `--ignore` flag).