Refactor ranking into a separate Mulkup.Ranking module.

Change-Id: Ieeef3f837756e6afe96f0a0c3dba591f285daae5
diff --git a/mulkup.cabal b/mulkup.cabal
index 82fc1a6..3c06e38 100644
--- a/mulkup.cabal
+++ b/mulkup.cabal
@@ -100,6 +100,7 @@
       Mulkup.Logging
       Mulkup.Main
       Mulkup.Prelude
+      Mulkup.Ranking
 
   hs-source-dirs:
       src