Orgize does not produce html image tags for image links

[JWorthe]

Good evening! Thank you for writing and maintaining this library. I think it's great to have a Rust library that can render Org-mode to html.

Some unexpected behaviour I've run into is that DefaultHtmlHandler doesn't render images. I think it's because images aren't actually mentioned in the spec, but are rather a special case for links to files with certain extensions.

For example, [[org-mode-unicorn.png]] should render to <img src="org-mode-unicorn.png" /> rather than an <a> tag because of the png extension.

This page describes how I would expect the default behaviour to be: https://orgmode.org/worg/org-tutorials/images-and-xhtml-export.html