Only the first code block is highlighted when using an async highlighter in 1.1.0

[qubyte]

Describe the bug
Only the first code block is highlighted when using an async highlighter in 1.1.0

To Reproduce
Steps to reproduce the behavior:

Consider the following simplified code:

var marked = require("marked"); // v1.1.0
var highlightjs = require("highlight.js");

// I realise this could be synchronous. It's asynchronous to
// demonstrate the problem.
function highlight(code, language, callback) {
  return callback(null, highlightjs.highlight(language, code).value);
}

const testString = [
  'Some text.',
  '```javascript',
  'function hello(a, b, c) {',
  '  console.log(a, b, c);',
  '}',
  '```',
  'Some more text.',
  '```javascript',
  'function hello(a, b, c) {',
  '  console.log(a, b, c);',
  '}',
  '```'
].join('\n');

marked(testString, { highlight }, (err, result) => console.log(result));

The resultant logged string is:

<p>Some text.</p>
<pre><code class="language-javascript"><span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">hello</span>(<span class="hljs-params">a, b, c</span>) </span>{
  <span class="hljs-built_in">console</span>.log(a, b, c);
}</code></pre>
<p>Some more text.</p>
<pre><code class="language-javascript">function hello(a, b, c) {
  console.log(a, b, c);
}</code></pre>

The first code snippet is highlighted, but the second (and all subsequent if you add more) codeblock is not. This only occurs when using an async highlighter.

Expected behavior
All code blocks should be highlighted. The expected output of the above snippet is:

<p>Some text.</p>
<pre><code class="language-javascript"><span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">hello</span>(<span class="hljs-params">a, b, c</span>) </span>{
  <span class="hljs-built_in">console</span>.log(a, b, c);
}</code></pre>
<p>Some more text.</p>
<pre><code class="language-javascript"><span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">hello</span>(<span class="hljs-params">a, b, c</span>) </span>{
  <span class="hljs-built_in">console</span>.log(a, b, c);
}</code></pre>

[UziTech]

It looks like it is because the callback gets called synchronously

This should fix it:

function highlight(code, language, callback) {
  setTimeout(() => {
    callback(null, highlightjs.highlight(language, code).value);
  }, 0);
}

[qubyte]

This issue actually flagged some old code which I needed to clean up (it can be synchronous now). Thanks for the tip though. 👍