code: function getCode(match, lang) {
    return match.substring(`<pre><code class="${lang}">`.length, match.indexOf('</code>'))
      .replace(/&#39;/g, '\'')
      .replace(/&lt;/g, '<')
      .replace(/&gt;/g, '>')
      .replace(/&amp;/g, '&')
      .replace(/&quot;/g, '"');
  }
docs: Gets the code within a <code></code> block. It also unescapes any html
  that was escaped by marked.
  
  @param {String} match - the current match.
  @param {String} lang - the current language.
  @return {String} the code within the <code></code> block.