Allow serving any renderer's output

[ISSOtm]

I use custom renderers, which serve to post-process the HTML renderer's output (this is arguably fragile, relying on implementation details of that renderer, but it seems to be the least bad solution) and some other files (such as stripping some "utility" comments from a file, in the latter's case).

One downside is not being able to use serve's auto-reload feature, which this adds support for.