Escape certain html tags in a string
<p>I need to escape all html tags but with some exception like: &lt;b>, &lt;font> etc</p>

<p>For example:</p>

<pre><code> Hello &lt;b&gt;world&lt;/b&gt;. How are &lt;span&gt;you&lt;/span&gt;?
</code></pre>

<p>Will result in: Hello <b>world</b>. How are you &lt;span>you&lt;/span></p>
