Implement plugin-in architecture with JavaScript/Node.js

Below is a simple node.js using express

var express = require('express');
var app = express();

app.get('/', function(req, res){
  res.send('Hello World');
});

app.listen(3000);

I want to implement a plugin-in architecture, such as by default there is a folder called plugins, and they register by themselve when the node.js startup, I don't need to modify the main server.js

an example a foo plugin, e.g.

PluginManager.register("init, function(app) {
    app.get('/foo', function(req, res){
        res.send('Hello from foo plugin');
    });
});

Then in my server.js, I would have something like

// TODO: Scan all scripts under the folder `plugins`
// TODO: Execute all the `init` hook before the app.listen
app.listen(3000);

My problem is my plugin codes are all asynchronous , there is no way to block before the app.listen(), do you have any idea for a better plugin-in architecture implementation?

Thanks.