Allow apps to specify extra body to be added to server side HTML output

This allows simple way of adding output to the server side HTML output. Additionally, it moves loading of (possibly big) JavaScript to the end of the body, so that body is displayed first in the browser, and then JavaScript is loaded. This does not change much when this API is not used, but changes usefulness a lot, when it is.

For example of its use, see https://peerlibrary.org/. Notice the "PeerLibrary is loading" message. This is not possible otherwise.

We are using it for [two things](https://github.com/peerlibrary/peerlibrary/blob/development/server/lib/body.coffee):
* loading message
* `<noscript>` message