Edit pruneCss flag to allow printing pruned css instead of overwriting.

[MonamAgarwal]

This will enable user to see the pruned CSS and them make changes according to their own requirements.

[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.

[ochafik]

As discussed, can you deprecate the existing flag and add a new one? I found 1 existing usage of pruneCss with a bool value, this would break it.

Alternatively, we can keep it and accept true, false, warnOnly

[MonamAgarwal]

Thanks Olivier.
Can you please point me to existing usage of pruneCss. Because AFAIS I have replaced all the usages. So this should work fine.

[ochafik]

Also, can you please document each enum value in the readme with the same format as the other enums?