Add Markdown as an output format

Add Markdown as an output format

The motivation behind this is not to make it easier to publish Markdown files, as that sounds unusual.

This is mainly meant for shortcodes that produces Markdown to be inlined.

You would do this by creating shortcodes with `*.md` suffix (e.g. `layouts/shortcodes/myshortcode.md`).

This output format is defined as plain text, and will use Go's much more lenient text template parser.

Updates #9821