Read the YAML metadata (and any common output YAML file) for the
document and return the output format that will be generated by
a call to render.
default_output_format(input, output_yaml = NULL)A named list with a name value containing the format
name and an options value that is a list containing all the options
for the format and their values. An option's default value will be returned
if the option isn't set explicitly in the document.
Input file (Rmd or plain markdown)
Paths to YAML files specifying output formats and their
configurations. The first existing one is used. If none are found, then
the function searches YAML files specified to the output_yaml top-level
parameter in the YAML front matter, _output.yml or _output.yaml, and then uses
the first existing one.
This function is useful for front-end tools that require additional
knowledge of the output to be produced by render (e.g. to
customize the preview experience).