Move code to Internal module and add tests

src/HCat/Internal.hs

@@ -0,0 +1,8 @@
+-- | Internal module in order to facilitate testability.
+module HCat.Internal where
+
+parseArgs :: [String] -> Either String FilePath
+parseArgs args = case args of
+    [] -> Left "No filename given!"
+    [arg] -> Right arg
+    _ -> Left "Only a single file is supported"