Run metacompilers instead of ignoring them

src/Hakyll/Core/Run.hs

@@ -141,8 +141,9 @@ runCompilers ((id', compiler) : compilers) = Hakyll $ do
     result <- liftIO $ runCompiler compiler id' provider url store isModified
     liftIO $ putStrLn $ "Generated target: " ++ show id'
 
-    let CompileRule compiled = result
-
+    case result of
+        -- Compile rule for one item, easy stuff
+        CompileRule compiled -> do
             case url of
                 Nothing -> return ()
                 Just r  -> liftIO $ do
@@ -155,3 +156,8 @@ runCompilers ((id', compiler) : compilers) = Hakyll $ do
 
             -- Continue for the remaining compilers
             unHakyll $ runCompilers compilers 
+
+        -- Metacompiler, slightly more complicated
+        MetaCompileRule newCompilers -> do
+            -- Actually I was just kidding, it's not hard at all
+            unHakyll $ addNewCompilers compilers newCompilers