Support permalinks with no extension or slash

Jekyll supports extensionless permalinks with no trailing slash [1],
as in the following:

  /year/month/slug

For such a permalink, Jekyll generates a file `/year/month/slug.html`
and assumes the web server will play nicely and serve this file when
it receives a request for `/year/month/slug`.

This patch adds support for such permalinks for files Pandoc-generated
files in other formats, with the generated URLs having a form
mirroring the generated HTML files:

  /year/month/slug.format

[1]: https://jekyllrb.com/docs/permalinks/#extensionless-permalinks