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.conf

@@ -829,6 +829,10 @@ remoteuser = username
 #
 #oauth2_client_id = YOUR_CLIENT_ID
 #oauth2_client_secret = YOUR_CLIENT_SECRET
+#
+# The return values must be bytes.
+#oauth2_client_id_eval = get_client_id("accountname")
+#oauth2_client_secret_eval = get_client_secret("accountname")
 
 # Specify the refresh token to use for the connection to the mail server.
 # Here's an example of a way to get a refresh token:
@@ -853,6 +857,10 @@ remoteuser = username
 #oauth2_access_token = ACCESS_TOKEN
 #oauth2_request_url = https://accounts.google.com/o/oauth2/token
 #oauth2_refresh_token = REFRESH_TOKEN
+#
+# The return values must be bytes.
+#oauth2_access_token_eval = get_access_token("accountname")
+#oauth2_refresh_token_eval = get_refresh_token("accountname")
 
 ########## Passwords