gnu: Add emacs-org-tree-slide.

Thanks, Pjotr & Ricardo!  :-)

* gnu/packages/emacs.scm (org-tree-slide): New variable.

gnu/packages/emacs.scm

@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2014 Taylan Ulrich Bayirli/Kammer <taylanbayirli@gmail.com>
-;;; Copyright © 2013, 2014, 2015, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2018 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2014, 2015, 2016 Mark H Weaver <mhw@netris.org>
 ;;; Copyright © 2014, 2015, 2016, 2017 Alex Kost <alezost@gmail.com>
 ;;; Copyright © 2015 Federico Beffa <beffa@fbengineering.ch>
@@ -6918,3 +6918,25 @@ supports multiple backends such as @code{vlc}, @code{mpg123},
 Groovy source files, REPL integration with run-groovy and Grails project
 navigation with the grails mode.")
     (license license:gpl3+)))
+
+(define-public org-tree-slide
+  (let ((commit "dff8f1a4a64c8dd0a1fde0b0131e2fe186747134")
+        (revision "0"))
+    (package
+      (name "emacs-org-tree-slide")
+      (version (git-version "0.1" revision commit))
+      (home-page "https://github.com/takaxp/org-tree-slide")
+      (source (origin
+                (method git-fetch)
+                (uri (git-reference (url home-page) (commit commit)))
+                (sha256
+                 (base32
+                  "153bg0x7ypla11pq51jmsgzfjklwwnrq56xgpbfhk1j16xwz9hyf"))
+                (file-name (git-file-name name version))))
+      (build-system emacs-build-system)
+      (synopsis "Presentation tool for org-mode")
+      (description
+       "Org-tree-slide provides a slideshow mode to view org-mode files.  Use
+@code{org-tree-slide-mode} to enter the slideshow mode, and then @kbd{C->} and
+@kbd{C-<} to jump to the next and previous slide.")
+      (license license:gpl3+))))