Adds shortcode `[catthumb]` to embed a thumbnail image for a category.
[catthumb id="CategoryID" link="true"]
will find the most recent post in the specified Category with a featured image and return the thumbnail for that image. If link="true" is specified, the image will link to the category specified.
The plugin will only search featured images, not attached images. It will not look for images embedded in the post itself, or other attached images. If a category does not have any posts with featured images, this plugin will not return a thumbnail image, even if the category has posts with attached images.