extract code from knitr vignette and source it
source_purl(input, output = file.path(tempdir(), paste0(basename(input),
".R")), documentation = 1L, ...)path to file as single character string
output file path, defaults to a file in a temporary name based
on input
single integer value passed on to knitr::purl. An
integer specifying the level of documentation to go the tangled script: 0
means pure code (discard all text chunks); 1 (default) means add the chunk
headers to code; 2 means add all text chunks to code as roxygen comments
further parameters passed to source