Add raco slideshow tool. (#14)

This is identical to the existing slideshow executable. Similar to
how raco scribble is identical to the standalone scribble
executable.

Author: LeifAndersen

slideshow-exe/slideshow/info.rkt

@@ -8,3 +8,8 @@
 (define binary-keep-files '("tutorial-show.rkt"
                             "initial-ones.rkt"
                             "examples"))
+
+(define raco-commands '(("slideshow"
+                         slideshow/start
+                         "render a Slideshow document"
+                         #f)))