An optional filter function that
receives the output_format, files_dir, and
input_lines. The filter can be called for it's
side-effects (e.g. rendering supporting files to the
files_dir) and can also return a modified format.
post_processor
An optional post-processor function
that receives the input_file, output_file,
and verbose parmaeters.
Value
An R Markdown output format definition that can be passed
to render.