Add SASL PLAIN authentication method

- this method isn't as deprecated as IMAP LOGIN;

 - it allows to keep hashed passwords on the server side;

 - it has the ability to specify that the remote identity
   is different from authenticating username, so it even
   can be useful in some cases (e.g., migrated mailboxes);
   configuration variable "remote_identity" was introduced
   to leverage this functionality.

From: Andreas Mack <andreas.mack@konsec.com>
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap.conf

@@ -353,6 +353,18 @@ ssl = yes
 # Specify the remote user name.
 remoteuser = username
 
+# Specify the user to be authorized as.  Sometimes we want to
+# authenticate with our login/password, but tell the server that we
+# really want to be treated as some other user; perhaps server will
+# allow us to do that (or, may be, not).  Some IMAP servers migrate
+# account names using this functionality: your credentials remain
+# intact, but remote identity changes.
+#
+# Currently this variable is used only for SASL PLAIN authentication
+# mechanism.
+#
+# remote_identity = authzuser
+
 # There are six ways to specify the password for the IMAP server:
 #
 # 1. No password at all specified in the config file.