error in your readme

```
gulp.task('serve-prod', serve({
    root: ['public', 'build'],
    port: 80,
    middleware: function(req, res) {
        // custom optional middleware
    }
})
```

This is a part your README, there is a error in middleware, it should be 

```
middleware: yourfunction()
```

or modify  [index.js#27](https://github.com/nkt/gulp-serve/blob/master/index.js#L27) as
```
config.middlewares.push(config.middleware());
```

Actually I think the better way is as my PR.