Adding first posted and last updated dates to articles

[JeffreyBLewis]

As it stands articles have dates that correspond to when the articles where written. It would be useful to also have an "Updated" date that showed the last time an article was "updated". Ideally this date would reflect the last time the information contained in the article actually changed. Good enough would be an updated date that reflected the last time the Rmd was compiled even that if that recompilation did not result in any change.