Expose default help printer function

[yurikhan]

Improves #1329.

[google-cla]

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).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[yurikhan]

This is a breaking change to the API. If needed, I think we can overload Initialize to accept either an argumentless function or a two-argument version. Please say so if this is necessary.

[LebedevRI]

Hmm yeah, the current implementation is indeed not what i would expect either :/
I think the proper solution would be to expose the internal::DefaultHelperPrintf(), and let the user call it when needed.

[yurikhan]

@LebedevRI You mean like this?

[LebedevRI]

@LebedevRI You mean like this?

Yep!

[yurikhan]

Not sure what the sanitizer checks are trying to tell me.

[dmah42]

the sanitizer builds are currently broken. i haven't had a chance to look into it.