YAML frontmatter for R Markdown to cause rmarkdown::render() to retain intermediate Markdown file for .R and .Rmd files, respectively

## yaml_frontmatter_r_github_document.yaml
#' ---
#' title: "Something fascinating"
#' author: "Jenny Bryan"
#' date: "`r format(Sys.Date())`"
#' output: github_document
#' ---

## yaml_frontmatter_r_keep_md.yml
#' ---
#' title: "Something fascinating"
#' author: "Jenny Bryan"
#' date: "`r format(Sys.Date())`"
#' output:
#'   html_document:
#'     keep_md: TRUE
#' ---

## yaml_frontmatter_rmd_github_document.yaml
---
title: "Something fascinating"
author: "Jenny Bryan"
date: "`r format(Sys.Date())`"
output: github_document
---

## yaml_frontmatter_rmd_keep_md.yml
---
title: "Something fascinating"
author: "Jenny Bryan"
date: "`r format(Sys.Date())`"
output:
  html_document:
    keep_md: TRUE
---