provide an easy way to copy vignettes

[sebkopf]

vignettes don't usually ship with devtools::install_github() unless explicitly compiled (which takes a long time) so this may not be an option until the package is on CRAN but it would be quite useful to have a way to copy all vignettes into the current working directory for users to play with, something along these lines (pseudocode):

iso_copy_reader_vignettes <- function(target_folder = ".", overwrite_existing = FALSE, remove_vignette_header = TRUE) {
  ...
}