Rmd detection ignores front matter

[renkun-ken]

When user types in the front matter of the R Markdown document, some language providers are triggered.

---
title: "Hello document"
---

For exmaple, hitting "enter" in the end of the above title line, the range formatting will be triggered and the front matter becomes:

---
  title:"Hello document"

---

This is no longer a valid YAML front matter.