Pass ... to vignette engines

[jimhester]

This allows you to pass additional arguments to vignette engines if desired. This can be used in the ... argument to tools::buildVignette() or by explicitly retrieving the vignette engine, e.g.

vignetteEngine("knitr::rmarkdown")$weave("file.Rmd", clean = TRUE)

We would use this in devtools so we could save the intermediate md files when building Rmd vignettes with devtools::build_vignettes()

[yihui]

Thanks!