No easy way to pass the output of a single simplate page through a filter

In Liberapay we have a `jinja2_html_jswrapped` renderer, it could be nice to be able to replace it with something like `via jinja2 | jswrap` in the simplate. `jswrap` would be a simple function, either defined/imported in the simplate's first page, or registered globally like renderers.
