The Custom Formatters module lets you easily create custom field formatters through an admin UI without having to create a custom module. Custom formatters are exported as config entities.
Stuart initially created this module back in 2009 and recently picked up active maintainership again with the new 4.1 branch.
In this article he shows us around the module's new architecture and features, including support for Devel Generate and a built-in twig code editor.