Add vmd support in markdown mode

layers/+lang/markdown/README.org

@@ -25,6 +25,7 @@ This layer adds markdown support to Spacemacs.
 
 ** Features:
 - markdown files support via [[http://jblevins.org/git/markdown-mode.git/][markdown-mode]]
+- Fast Github-flavored live preview via [[https://github.com/blak3mill3r/vmd-mode][vmd-mode]]
 - TOC generation via [[https://github.com/ardumont/markdown-toc][markdown-toc]]
 - Completion of Emojis using [[https://github.com/dunn/company-emoji][company-emoji]] (still needs a way of showing, either
   using the =emoji= layer or having a proper font) :clap:
@@ -38,6 +39,9 @@ To use this configuration layer, add it to your =~/.spacemacs=. You will need to
 add =markdown= to the existing =dotspacemacs-configuration-layers= list in this
 file.
 
+To enable Github-flavored live preview, you will need to install the executable
+with =npm install -g vmd=.
+
 * Usage
 To generate a table of contents type on top of the buffer:
 ~SPC SPC markdown-toc/generate-toc RET~
@@ -117,6 +121,7 @@ To generate a table of contents type on top of the buffer:
 | ~SPC m c m~ | other window                            |
 | ~SPC m c p~ | preview                                 |
 | ~SPC m c P~ | live preview in Emacs' built-in browser |
+| ~SPC m c d~ | Github-flavored live preview            |
 | ~SPC m c e~ | export                                  |
 | ~SPC m c v~ | export and preview                      |
 | ~SPC m c o~ | open                                    |

layers/+lang/markdown/packages.el

@@ -19,6 +19,7 @@
     markdown-toc
     mmm-mode
     smartparens
+    vmd-mode
     ))
 
 (defun markdown/post-init-company ()
@@ -216,3 +217,13 @@ Will work on both org-mode and any mode that accepts plain html."
       (mmm-add-mode-ext-class 'markdown-mode nil 'markdown-javascript)
       (mmm-add-mode-ext-class 'markdown-mode nil 'markdown-ess)
       (mmm-add-mode-ext-class 'markdown-mode nil 'markdown-rust))))
+
+(defun markdown/init-vmd-mode ()
+  (use-package vmd-mode
+    :if (executable-find "vmd")
+    :defer t
+    :init
+    (progn
+      (spacemacs/set-leader-keys-for-major-mode 'markdown-mode
+        ;; Github-flavored live preview in a separate window
+        "cd" 'vmd-mode))))