How do i dynamically require module, instantiate object and call function in JS

Building a node app where I'm consistently requiring and doing things with several different bitcoin exchange modules that I've built. Sometimes I want to add or remove an exchange and it's painful to go through the code to add and remove all the hard-coded references to the exchanges.

Is there a way to do something like this? Sorry if this is an obvious question - I'm relatively new to node and a (programming) weekend warrior generally. Thanks in advance for your input.

//config.js    
var config = {}
config.exchanges = ['bitfinex', 'bitstamp', 'btce'];
module.exports = config;

//app.js
var config = require('./config');
var _= require('underscore');

_.each(config.exchanges, function(exchange){

    //obviously not correct, but how would I accomplish something like this?

    var Exchange = require('./api/exchange');
    var exchange = new Exchange();
    exchange.do_stuff();

});