Shortcode to display category-specific coupons
While we already have a shortcode to display all created coupons, it'd be nice to have additional arguments to only display coupons that apply to specific categories. That way, we can create a catalogue of coupons to be displayed on separate pages. And maybe also add them to product category pages, that way users can browse these categorized items and see the coupons that are available for such category.
