Install markdown-it-ruby

Signed-off-by: Yukai Huang <yukaihuangtw@gmail.com>

package.json

@@ -170,6 +170,7 @@
     "jsonlint": "~1.6.2",
     "less": "~3.9.0",
     "less-loader": "~4.1.0",
+    "markdown-it-ruby": "^0.1.1",
     "mini-css-extract-plugin": "~0.4.1",
     "mocha": "~5.2.0",
     "mock-require": "~3.0.3",

public/js/extra.js

@@ -1001,6 +1001,7 @@ md.use(require('markdown-it-mathjax')({
   afterDisplayMath: '\\]</span>'
 }))
 md.use(require('markdown-it-imsize'))
+md.use(require('markdown-it-ruby'))
 
 md.use(require('markdown-it-emoji'), {
   shortcuts: {}