Issue title: Provide option to export as beamer pdf using pandoc.
 Issue body: 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.

 Comments: 
 Comment 0: Here are some ideas.

``` R
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
  }
}
```

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

{'source': 'GithubIssues', 'repo_name_with_owner': 'ramnathv/slidify', 'issue_id': 5004299, 'repo_stars': 848.0}