protect html callout tag

[Alexis Krier]

Hi, in my code I have “callout”

here is my html before highlight processing:

```

<pre class="highlightjs highlight">
<code class="language-ruby hljs" data-lang="ruby">require 'sinatra' <img src="http://mrte.rc.free.fr/font-awesome-4.7.0PNG/callouts/1.png" alt="1">
```

then I call 

```

hljs.highlightAuto(myhtmlblock)
```

and then my img callout sisapear, do you know how to keep it?

```

<pre class="highlightjs highlight"><code class="language-ruby hljs" data-lang="ruby">
<span class="hljs-keyword">require</span> 
<span class="hljs-string">'sinatra'</span>
```