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/repository/Base.py

@@ -259,3 +259,8 @@ class BaseRepository(CustomConfig.ConfigHelperMixin, object):
         for the threads to terminate."""
         pass
 
+    def getlocalroot(self):
+    	""" Local root folder for storing messages.
+    	Will not be set for remote repositories."""
+        return None
+