Provide option to export as beamer pdf using pandoc.

[ramnathv]

In order to enable this option, I need to follow the guidelines for slide formatting in pandoc. Here are the key things
to do in order to make slidify decks pandoc compatible

1. Remove class definitions from slide separators.
2. Download and replace online images with local images.
3. Remove content with iframe, script etc.

[ramnathv]

Here are some ideas.

pandoc <- function(..., path = 'pandoc', cmdOnly = FALSE){
  xargs <- list(...)
  nxargs <- names(xargs)
  nxargs <-ifelse(nxargs == "", "", sprintf("--%s=", nxargs))
  xcmd  <- paste(sprintf('%s%s', nxargs, xargs), collapse = " ")
  if (!cmdOnly) {
    system(sprintf('%s %s', path, xcmd))
  } else {
    xcmd
  }
}

pandoc(write = "beamer", "SLIDES.txt", output = 'example9.pdf', 
  cmdOnly = FALSE, variable="theme:CambridgeUS")