curl: added basic SASL XOAUTH2 support

Added the ability to specify an XOAUTH2 bearer token [RFC6750] via the
--bearer option.

Example usage:
  curl --url "imaps://imap.gmail.com:993/INBOX/;UID=1" --ssl-reqd
  --bearer ya29.AHES6Z...OMfsHYI --user username@example.com
curl: added --bearer option to help

Added the --bearer option to the help output