add "?" as the argCount to commands with an optional single arg

content/io.js

@@ -367,6 +367,7 @@ function IO() //{{{
             autocommands.trigger("ShellCmdPost", {});
         },
         {
+            argCount: "?", // TODO: "1" - probably not worth supporting weird Vim edge cases. The dream is dead. --djk
             bang: true,
             completer: function (context) completion.shellCommand(context),
             literal: 0