Postprocessors: Make it possible to specify next or stack of postprocessors

[HenrikBengtsson]

Example:

html <- R.rsp::rfile("main.Rmd.rsp", postprocess = TRUE)  # default
html <- R.rsp::rfile("main.Rmd.rsp", postprocess = "knitr")  # which the default uses
html <- R.rsp::rfile("main.Rmd.rsp", postprocess = "Rmarkdown")

Right now the latter can be achieved manually by:

html <- rmarkdown::render(R.rsp::rfile("main.Rmd.rsp", postprocess = FALSE))

[HenrikBengtsson]

Another example is

html <- R.rsp::rfile("main.md.rsp", postprocess = TRUE)  # default
html <- R.rsp::rfile("main.md.rsp", postprocess = "markdown")  # same as the default
html <- R.rsp::rfile("main.md.rsp", postprocess = "commonmark")