Make waitUntil a config driven option

[Fabian Gargaglione]

In some cases users will want to change the waitUntil configuration from puppeteer to avoid timeouts with firebase sockets or related open connection that might be left in some type of applications.

It would a very easy enhancement, and in my case would solve a timeout issue with Rendertron.
There should be no point to keeping this configuration as a hardcoded value on the app.

[Martin Splitt]

In general, as we're deprecating the project, you should look into alternative approaches to rendering on the web.