Fix detecting yaml frontmatter

[renkun-ken]

What problem did you solve?

Closes #855

This PR fixes the detection of yaml frontmatter in rmd. It only tries to capture the first yaml chunk surrounded by --- at the top of the document.

(If you do not have screenshot) How can I check this pull request?

Knit the following document should use bookdown::render_book instead of rmarkdown::render.

---
output: bookdown::gitbook
knit: bookdown::render_book
---

# Test

Para.

---

# Next

[ElianHugh]

LGTM, didn't run into any errors. Thanks!

(Not sure why the docker build failed)