fix: title cannot be number at front matter (vuejs#297)

Author: ulivz

lib/util/parseHeaders.js

@@ -1,16 +1,16 @@
 const parseEmojis = str => {
  const emojiData = require('markdown-it-emoji/lib/data/full.json')
- return str.replace(/:(.+?):/g, (placeholder, key) => emojiData[key] || placeholder)
+ return String(str).replace(/:(.+?):/g, (placeholder, key) => emojiData[key] || placeholder)
 }
 
-const unescapeHtml = html => html
+const unescapeHtml = html => String(html)
  .replace(/"/g, '"')
  .replace(/'/g, '\'')
  .replace(/:/g, ':')
  .replace(/&lt;/g, '<')
  .replace(/&gt;/g, '>')
 
-const removeMarkdownToken = str => str
+const removeMarkdownToken = str => String(str)
  .replace(/`(.*)`/, '$1') // ``
  .replace(/\[(.*)\]\(.*\)/, '$1') // []()
  .replace(/\*\*(.*)\*\*/, '$1') // **