show option usage help with invalid `—node` option and simplied the valid option check to pure regex

cmd/cmd.js

@@ -278,6 +278,7 @@ class Cmd {
         const urlRegexExp = /^(vm|embark|((ws|https?):\/\/([a-zA-Z0-9_.-]*):?([0-9]*)?))$/i;
         if (!(node === 'embark' || node === 'vm' || node.match(urlRegexExp))) {
           console.error(`invalid --node option: must be 'vm', 'embark' or a valid URL`.red);
+          options.outputHelp();
           process.exit(1);
         }
         options.node = node;