fix: use header's slug as it is if possible (#119)

lib/util/index.js

@@ -59,10 +59,11 @@ exports.extractHeaders = (content, include = [], md) => {
   tokens.forEach((t, i) => {
     if (t.type === 'heading_open' && include.includes(t.tag)) {
       const title = tokens[i + 1].content
+      const slug = t.attrs.find(([name]) => name === 'id')[1]
       res.push({
         level: parseInt(t.tag.slice(1), 10),
         title,
-        slug: md.slugify(title)
+        slug: slug || md.slugify(title)
       })
     }
   })