Add a server option to get a reference to the dev server when ready

It can be useful in some circumstances to get a reference to the HTTP server in order to be able to programmatically request a client refresh.
Maybe something like:
```js
eleventyConfig.setServerOptions({
    ....
    ready: (ds) => {
        initCustomWatcher(eleventyConfig, ds);
    }
    ....
}
```
and then somewhere else: `ds.sendUpdateNotification(...)`.
