Fix(cli): Hide error for argument of help

The previous commit means that if the user wrote:
  `canonical_data_syncer help`

The program printed:
  Error: invalid argument: 'help'

and then showed the help message. This commit hides that error, so that
only the help message is shown for this special case.

See: exercism#35

src/cli.nim

@@ -102,6 +102,10 @@ proc parseArguments*: Arguments =
       else:
         showError(&"invalid option: '{kind.prefix}{key}'")
     of cmdArgument:
+      case key.toLowerAscii
+      of HelpArgument.long:
+        showHelp()
+      else:
         showError(&"invalid argument: '{key}'")
     of cmdEnd:
       discard