Update index.js, adding new option.

This option have name access and it's for security of given static files. This option can be true if user give Boolean true or dont give anything or false if user give Boolean false or anything else. Examples below:

app.use(express.static(__dirname + '/public', {access: true} )); // show folder
app.use(express.static(__dirname + '/public' )); // show folder
app.use(express.static(__dirname + '/public', {access: false} )); // do not show folder
app.use(express.static(__dirname + '/public', {access: 'asdqw2'} )); // do not show folder