offlineimap.conf: learn to evaluate oauth2 related options

Introduce:
- oauth2_client_id_eval
- oauth2_client_secret_eval
- oauth2_access_token_eval
- oauth2_refresh_token_eval

Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/307
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/CustomConfig.py

@@ -16,8 +16,8 @@
 
 import os
 import re
-import six
 from sys import exc_info
+import six
 
 try:
     from ConfigParser import SafeConfigParser, Error