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/IMAP.py

@@ -201,11 +201,11 @@ class IMAPRepository(BaseRepository):
 
     def getsslcacertfile(self):
         """Return the absolute path of the CA certfile to use, if any"""
-        cacertfile = self.getconf('sslcacertfile', get_os_sslcertfile())
+        xforms = [os.path.expanduser, os.path.abspath]
+        cacertfile = self.getconf_xform('sslcacertfile', xforms,
+          get_os_sslcertfile())
         if cacertfile is None:
             return None
-        cacertfile = os.path.expanduser(cacertfile)
-        cacertfile = os.path.abspath(cacertfile)
         if not os.path.isfile(cacertfile):
             raise SyntaxWarning("CA certfile for repository '%s' could "
                                 "not be found. No such file: '%s'" \

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)