Allow an alternative ssr middleware to be used

[matthewp]

This makes it possibly to specify your own ssr middleware by providing the ssr option like so:

const server = serve(5050, {
    path: __dirname,
    proxy: 'http://localhost:6060',
    ssr: function(steal, options) {
        Do whatever you want here!
        return function(req, res) {

        };
    }
});