feat(config): extend arrays when merging config

[LoveIsGrief]

By default _.merge incrementally overwrites array when merging.
This can lead to surprising behavior when default options exist.

This will let values be added to existing arrays.

Closes #246

Please make sure that your commit messages follow our convention!

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[googlebot]

CLAs look good, thanks!