Add a callback to skip the opening of a recursive directory

[Amatewasu]

This PR allows passing a callback to say whether we want to open this directory based on the developer criteria (such as the name for example). For example, you can skip the directory with a name beginning with a point ('.git', '.backup', etc.).

[google-cla[bot]]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[tomayac]

Thank you, this looks like it could go in the right direction! Can you add the same logic to the legacy fallback implementation for consistency, please?

[google-cla[bot]]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[Amatewasu]

Sure! What do you think of the implementation?

For the legacy fallback, it is far from being fully optimized because the browser still has to load all the files. Moreover, the callback skipDirectory will be called several times for every folder (but this computation time is negligible compared to the files load time).

[tomayac]

Getting there, see my remarks. Can I also ask you to run npm run fix and commit the result, please?

[Amatewasu]

Sure thank you for your remarks, I committed the result.

[tomayac]

(Sorry for the delay, I'm busy with Chrome Dev Summit and other things, but I will get back to you, promised.)

[tomayac]

Thank you again! This has now been published as v0.21.1.