automatically take file modification time as rmarkdown date

[maxheld83]

this works by placing in the yaml:

date: "`r format(file.info('setup_bibliography.Rmd')$mtime, '%d %B, %Y')`"

But that's a mouthful and you have to add the file name manually, which seems unecessary.

Would be a small nice thing to have a shortcut R function that does this instead.
It basically just needs to figure out what "this" file currently is, and then run the above for it.

Would be nice to add this as a feature to knitr or rmarkdown, or it may already exist therein.

Might also take different kinds of dates such as releases or whatever.

[maxheld83]

if this were an actual PR to knitr or rmarkdown it'd be graded-5, for sure.