html_notebook format,
through the output_source function attached to a
output_format.
html_notebook_output_html(html, meta = NULL)
html_notebook_output_img(path = NULL, bytes = NULL, attributes = NULL, meta = NULL)
html_notebook_output_png(path = NULL, bytes = NULL, attributes = NULL, meta = NULL)
html_notebook_output_code(code, attributes = list(class = "r"), meta = NULL)path
and bytes, if bytes is NULL, the bytewise contents
will be obtained by reading the file.html_notebook, see http://rmarkdown.rstudio.com/r_notebook_format.html.