Introduce CustomConfig method that applies set of transforms

It is a bit cleaner than making chains of calls like
{{{
value = os.path.expanduser(value)
value = os.path.abspath(value)
}}}
since we do see all transformations to be applied in a single
iterable and have no repeated code like in the above example.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/repository/Maildir.py

@@ -61,7 +61,8 @@ class MaildirRepository(BaseRepository):
             os.utime(cur_dir, (cur_atime, os.path.getmtime(cur_dir)))
 
     def getlocalroot(self):
-        return os.path.expanduser(self.getconf('localfolders'))
+        xforms = [os.path.expanduser]
+        return self.getconf_xform('localfolders', xforms)
 
     def debug(self, msg):
         self.ui.debug('maildir', msg)