fix(CliConfig): allow set on array item properties

[platosha]

Support changing properties of array items in CliConfig.set method.
This makes the following commands to work:

• $ ng set apps[0].main my-main.ts
• $ ng set apps.0.test my-test.ts

Closes #1763

[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.

[hansl]

I'm in process of refactoring config, so I won't let that in. My PR should be ready by EOD.

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}