Add hook to generate the lastmod of posts

_plugins/posts-lastmod-hook.rb

@@ -0,0 +1,11 @@
+# Check for changed posts
+Jekyll::Hooks.register :posts, :post_init do |post|
+
+  commit_num = `git rev-list --count HEAD "#{ post.path }"`
+
+  if commit_num.to_i > 1
+    lastmod_date = `git log -1 --pretty="%ad" --date=iso "#{ post.path }"`
+    post.data['lastmod'] = lastmod_date
+  end
+
+end