Refactor ranking into a separate Mulkup.Ranking module.

Change-Id: Ieeef3f837756e6afe96f0a0c3dba591f285daae5
diff --git a/src/Mulkup/Config.hs b/src/Mulkup/Config.hs
index f3d9fee..4be5b64 100644
--- a/src/Mulkup/Config.hs
+++ b/src/Mulkup/Config.hs
@@ -1,6 +1,6 @@
 {-# LANGUAGE UndecidableInstances #-}
 
-module Mulkup.Config (TierConfig (..), MulkupConfig (..), readConfig) where
+module Mulkup.Config (TierConfig (..), TierConfigs (..), StashConfig (..), MulkupConfig (..), readConfig) where
 
 import Dhall
 import Mulkup.Prelude