Enable specifying arbitrary yaml files as alternatives of _output.yml

[atusy]

This PR enables render() and YAML front matter to specify arbitrary yaml files as alternatives of _output.yml or _output.yaml, and closes #1613 .

render() supports it by the output_yaml argument and YAML front matter supports it by the output_yaml top level parameter.
There, multiple yaml files can be specified and the first existing one is used.
If output_yaml is specified both for render and YAML front matter, then render has the priority.
If none are found, then _output.yml or _output.yaml will be used if they exists.

Example

Prepare bar.yaml

list(
  html_document = list(toc = TRUE)
) %>%
  yaml::as.yaml() %>%
  writeLines("bar.yml")

Prepare test-yaml.Rmd

---
output: html_document
output_yaml:
  - foo.yml
  - bar.yml
---

# A

Render Rmd

If simply rendered by

render("test-yaml.Rmd")

then, bar.yml is used because foo.yml is missing.

Even if output_yaml is commented out from the YAML frontmatter, bar.yml can still be used via render().

render(
  "test-yaml.Rmd",
  output_yaml = "bar.yml"
)

[yihui]

Looks good! Just nitpicking: could you change yaml to YAML in text (including comments)? Thanks!

[atusy]

Thanks a lot. Commited your suggestions and also fixed a grammar mistake.

[atusy]

Oh I need to update docs as well.

[atusy]

About the comments, I see lower case "yaml" in those not only edited by me.
Do you want me to fix them as well?

rmarkdown/R/output_format.R

Line 380 in 9611ee4

# yaml front matter discovery is done within render)

[yihui]

I don't want to bring too much trivial work to you. Let's leave other instances of yaml behind. If you do want to change them, please feel free to do so in another PR. I don't really care. As I said, I was only nitpicking.

Thank you!