This rebuilds outdated vignettes with tools::buildVignette.
By default, it will rebuild all vignettes if the source file is newer than
the output pdf or html. (This means it will automatically re-build the
vignette if you change the vignette source, but not when you
change the R code). If you want finer control, add a Makefile to
vignettes/
and roxygen2 will use that instead.