Use new addition in pre_knit for metadata processing

[cderv]

rticles/R/article.R

Lines 616 to 618 in fcbcf83

	# for backward compatibility as we changed the template in
	# https://github.com/rstudio/rticles/pull/494
	options <- rmarkdown::yaml_front_matter(input)

See rstudio/rmarkdown#2485