This is a nice feature added to a great project. It's used in Whaleshares.io to let users add emoji's into their posts. Adding this feature means the users can load in a smaller spritesheet, instead of individual files.
What feature(s) did you add?
One thing I needed on a project was use of spritesheets for the nodejs library for emoji-mart. One of my collegues suggested and made a github issue. So I decided on my off time to work on it, and I made a merge request.
How did you implement it/them?
The main thing I did was allow for a seperate value for the Custom Emoji object called spriteUrl. This puts it as a background image, and everything is already built in.
While it was simple, it will allow a lot of users to easily use spritsheets for emoji-mart.