This is a game created with MakeRetroGames! MakeRetroGames is an interactive game creation tool that helps you build your own game, with source.
The tool is currently in a closed alpha test - if you're interested, please send an email to admin@cpprograms.net.
ps: If you'd like to use this without the tool, go for it. However, I would strongly suggest considering nes-starter-kit instead. It has a guide on how to make all sorts of changes, and much less editor quirkiness.