search: Pass debug output from mockup to stderr

Allows the mockup to easily print debugging information by prefixing
each line with "debug".

src/Search/Search.hs

@@ -192,7 +192,9 @@ parseLine handle = do
     then return EOF
     else do
       line <- T.hGetLine handle
-      case decodeStrict (T.encodeUtf8 line) of
+      if "debug" `T.isPrefixOf` line then
+        T.hPutStrLn stderr line >> parseLine handle
+      else case decodeStrict (T.encodeUtf8 line) of
         Nothing -> error $ "Parser error in line " <> T.unpack line
         Just (DecisionRequest num) ->
           return $ Decision num