Print plugin support

Since getting the 'print-pdf' feature in I wanted to leverage the existing print-pdf plugin script.

I'm proposing of adding a new command option: "--print" or "-r" (since "-p" was reserved for port). It's fairly straightforward and I added to the README.

Also works when no initial path is sent to `reveal-md` cli but is a little silly since it will create a PDF of the directory listing of markdown files.