Consistent output destination directory structure

Problem

Currently if you use single renderer or multiple renderer the structure of the destination directory changes.

If you have a single renderer the output goes directly into the destination directory.

If you have multiple renderers a top level directory is created for each renderer.

This behavior makes it difficult to maintain and optional renderers.

The following creates DEST/index.html:

[output.html]

The following creates DEST/html/index.html:

[output.html]
[output.pdf]

Proposed Solution

I suggest to support additional configuraiton in the output, in this case dest to enable the user to specify where to put the files, keeping backward compatibility when empty and there are more than one is to put render name.

[output.html]
dest=html

Notes

Workaround, always specify both, when we need only html, set pdf command to true.

[output.html]
[output.pdf]
command = true

This is ugly...

Start by tracing how the output.html and output.pdf configuration is parsed and how renderer destination paths are chosen for single versus multiple renderers. Add support for an optional dest setting while preserving the current default behavior, then verify that the examples produce the requested directory structure.