Option to produce paths and filenames that match one-to-one with the sources

[nthiery]

Thanks to #670 and site.options.folders, it is now possible to produce a web site whose folder structure matches the folder structure of the source directory. However there remains some directory / file name rewriting:

• file and directory names are rewritten to lowercase
• accented characters are stripped: e.g. combinatoire-énumérative is rewritten to combinatoire-num-rative

Proposal

When site.options.folders is on (or possibly with an additional option), ensure a one-to-one correspondence for folders and file names between the sources and the produced site.

Motivation:

• Make it simpler for users and tooling to retrieve and edit the original source file.
• Ease transition from Jupyter-Book by preserving the site layout, and thereby URL pointing to specific resources in the site.

Thanks in advance!