Basic middleware support

Since configuring middleware requires to actually write code, I added a
simple mechanism to load custom pre-configured middleware. The
configuration file has a new `server.stack` setting and now looks like
that:

    {
        "server": {
            // default stuff omitted for brevity

            "stack": [
                "/path/to/module.js"
            ]
        }
    }

I'm not sure how `require` actually works, so right now I'd recommend
having absolute paths.

Each configured module should export a function that takes two argument
(the Worker instance and the request) and must return an array of
middlewares, for example:

    module.exports = function(worker, req) {
        return [
            function(req, res, next) {
                console.log('Hi I\'m a painfuly useless middleware');
                next();
            }
        ];
    });