Add support for OAuth authentication

[robstewart57]

Hi @spencerjanssen,

Would you accept this pull request, which I submitted against the repository that previously maintained this library?

jhickner/smtp-mail#38

This change adds sendMailWithLoginOAuthSTARTTLS to the Network.Mail.SMTP module.

[spencerjanssen]

Yes, that change is welcome. Would you please include in the PR:

• A version bump to 0.5.0.0 (due to the new constructor in AuthType)
• An entry in CHANGELOG.md for 0.5.0.0 describing your changes

If you happen to have any ideas on how to test OAuth in CI that would be cool too, but that isn't a blocker by any means.

Cheers!

[robstewart57]

Hi @spencerjanssen ,

I've created a pull request: #23

[spencerjanssen]

Completed in #23