Further widgets integration with ACF

[Marek Lewandowski]

There are some cases that we want to prevent ACF from removing something, which may appear to be junk html.

Consider following UC for media embed plugin:
Gist uses spans with custom classes for syntax highlighting, for instance:

<div class="line" id="LC204">
    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
    <span class="nb">document</span>
    <span class="p">.</span>
    <span class="nx">removeEventListener</span>
    <span class="p">(</span>
    <span class="s1">'DOMContentLoaded'</span>
    <span class="p">,</span> 
    <span class="nx">onReady</span>
    <span class="p">,</span> 
    <span class="kc">false</span> 
    <span class="p">);</span>
</div>

We want to keep classes for spans.