[org] Add epub support

Toggle variable named `org-enable-epub-support` lets user enable/disable epub
support.

CHANGELOG.develop

@@ -1743,6 +1743,7 @@ Other:
 - Added the org-mode keybindings to the org-journal major mode (thanks to Marty
   Buchaus)
 - Replace ox-reveal with org-re-reveal (thanks to Magnus Therning)
+- Added epub support (thanks to vishvanath45)
 **** Osx
 - Fixed OSX mapping issue (thanks to Joey Liu)
 - Added layer variables to customize modifier behaviors on macOS:

layers/+emacs/org/README.org

@@ -20,6 +20,7 @@
   - [[#project-support][Project support]]
   - [[#org-brain-support][Org-brain support]]
   - [[#mode-line-support][Mode line support]]
+  - [[#epub-support][Epub support]]
 - [[#key-bindings][Key bindings]]
   - [[#starting-org-mode][Starting org-mode]]
   - [[#toggles][Toggles]]
@@ -290,6 +291,16 @@ To permanently enable mode line display of org clock, add this snippet to your
   (setq spaceline-org-clock-p t)
 #+END_SRC
 
+** Epub support
+To install the Org exporter [[https://github.com/ofosos/ox-epub][ox-epub]] that generates e-book file format [[https://en.wikipedia.org/wiki/EPUB][epub]], set
+the variable =org-enable-epub-support= to =t=.
+
+#+BEGIN_SRC emacs-lisp
+  (setq-default dotspacemacs--configuration-layers
+                '((org :variables
+                       org-enable-epub-support t)))
+#+END_SRC
+
 * Key bindings
 ** Starting org-mode
 

layers/+emacs/org/packages.el

@@ -39,6 +39,7 @@
         persp-mode
         (ox-hugo :toggle org-enable-hugo-support)
         (org-trello :toggle org-enable-trello-support)
+        (ox-epub :toggle org-enable-epub-support)
         ))
 
 (defun org/post-init-company ()
@@ -715,3 +716,7 @@ Headline^^            Visit entry^^               Filter^^                    Da
         "mtdc" 'spacemacs/org-trello-pull-card
         "mtub" 'spacemacs/org-trello-push-buffer
         "mtuc" 'spacemacs/org-trello-push-card))))
+
+(defun org/init-ox-epub ()
+  (use-package ox-epub
+    :after ox))