initialise app with a file path, for the functionality of loading different files from the terminal

imperative-shell/shell.sml

@@ -3,7 +3,8 @@ struct
   fun main () =
     case CommandLine.arguments () of
       ["-r"] => Converter.main ()
-    | [] => InitGlfw.main ()
+    | [filename] => InitGlfw.main filename
+    | [] => print "error: no arguments\n"
     | args =>
         let
           val args = String.concatWith "" args