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

lib/util/parseHeaders.js

 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')  // **