Add support for XOAUTH2 SASL authentication

[rlebeau]

Outlook/Hotmail/Live, Gmail, and possibly others, support XOAUTH2 authentication over SASL for POP3, SMTP and IMAP. Indy should implement a TIdSASL component to support this. This way, users do not need to create application-specific passwords when 2-step verification is enabled in their accounts.

https://developers.google.com/gmail/imap/xoauth2-protocol

https://msdn.microsoft.com/en-us/library/dn440163.aspx