app.js

var config = require('./config');

config.js

var _ = require('lodash'),
    fs = require('fs');

exports.server_host = process.env.SERVER_HOST || 'localhost';

if (require('fs').existsSync('./local.config.js')) {
    _.extend(exports, require('./local.config.js'));
}

local.config.js

exports.override = 'stuff here!';