Add new expansion key for mbnames.peritem config variable

It is called localfolders and holds expanded name for the same
variable for the local repository of the account that is being
processed.

GitHub issue: https://github.com/OfflineIMAP/offlineimap/issues/21
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap.conf

@@ -129,6 +129,12 @@ accounts = Test
 #
 # The header, peritem, sep, and footer are all Python expressions passed
 # through eval, so you can (and must) use Python quoting.
+#
+# The following hash key are available to the expansion for 'peritem':
+# - accountname: the name of the corresponding account;
+# - foldername: the name of the folder;
+# - localfolders: path to the local directory hosting all Maildir
+#   folders for the account.
 
 enabled = no
 filename = ~/Mutt/muttrc.mailboxes