feat: underline markdown support (#2073)

* fix: no markdown support for underline #2072

server/modules/rendering/markdown-core/definition.yml

@@ -34,12 +34,19 @@ props:
     hint: Enable some language-neutral replacement + quotes beautification
     order: 4
     public: true
+  underline:
+    type: Boolean
+    default: false
+    title: Underline Support
+    hint: Enable underline by using _underline_
+    order: 5
+    public: true
   quotes:
     type: String
     default: English
     title: Quotes style
     hint: When typographer is enabled. Double + single quotes replacement pairs. e.g. «»„“ for Russian, „“‚‘ for German, etc.
-    order: 5
+    order: 6
     enum:
       - Chinese
       - English

server/modules/rendering/markdown-core/renderer.js

@@ -1,6 +1,7 @@
 const md = require('markdown-it')
 const mdAttrs = require('markdown-it-attrs')
 const _ = require('lodash')
+const underline = require('./underline')
 
 const quoteStyles = {
   Chinese: '””‘’',
@@ -30,6 +31,10 @@ module.exports = {
       }
     })
 
+    if (this.config.underline) {
+      mkdown.use(underline)
+    }
+
     mkdown.use(mdAttrs, {
       allowedAttributes: ['id', 'class', 'target']
     })

server/modules/rendering/markdown-core/underline.js

@@ -0,0 +1,12 @@
+const renderEm = (tokens, idx, opts, env, slf) => {
+  const token = tokens[idx];
+  if (token.markup === '_') {
+    token.tag = 'u';
+  }
+  return slf.renderToken(tokens, idx, opts);
+}
+
+module.exports = (md) => {
+  md.renderer.rules.em_open = renderEm;
+  md.renderer.rules.em_close = renderEm;
+}