Support private email address that are not in api responses

[antonydenyer]

Fixes #41

[mikatuo]

Thanks for the fix!
IMHO it's better to return nil for private/not-available emails. It would be easier to check if the user has an email if it's not nil, rather than comparing with id+username@users.noreply.github.com. It makes sense to ask a user for a valid email in this case.

[antonydenyer]

@mikatuo I think that has some knock-on effects for the rest of ueberauth.

[yordis]

@ueberauth/core could you take a look to this one

[grrrisu]

Hello
I just set the allow_private_emails flag, but had no effect. Until I realized, that this feature has never been released. Any intension to publish this?

[yordis]

@grrrisu Done