Feature request - implement "passthrough" option to preserve selected elements

Using markdownify 0.11.6, and it is working like a charm except for one thing. I'm scraping a site that has a youtube video embedded in an iframe. In this case i need to just it unchanged from out the site had it.

an option like --skip 'iframe' for example would be great. (ideally with some criteria, such as matching an id or regex).

The following change produces the desired outcome. It's obviously just a quick hack, but it demonstrates to the functionality.

in init.py line ~143

        for el in node.children:
            if isinstance(el, Comment) or isinstance(el, Doctype):
                continue
            elif isinstance(el, NavigableString):
                text += self.process_text(el)
            else:
                if el.name == 'iframe':
                    text += self.process_text(el)
                else:
                    text += self.process_tag(el, convert_children_as_inline)

test.html

<p>Need a way to preserve the original html for a given element.</p>
<i>Please don't discard my iframe :) </i>
<div class="ratio ratio-16x9" data-video="">
    <iframe allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" class="border" data-video="" src="https://www.youtube.com/embed/EHfq0miBu8c?modestbranding=0&amp;rel=0&amp;showinfo=0"></iframe>
</div>
<hr />

Produces:

===================


Need a way to preserve the original html for a given element.


*Please don't discard my iframe :)* 

<iframe allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" class="border" data-video="" src="https://www.youtube.com/embed/EHfq0miBu8c?modestbranding=0&amp;rel=0&amp;showinfo=0"></iframe>



---```

Start in init.py around line 143, where child elements are converted, and review how the supplied test.html example is processed. Define how a passthrough selection option should identify elements, then verify that selected elements such as the iframe retain their original HTML while other content is still converted.