Add itch.io release + executable binaries to Makefile

[vm70]

The Makefile currently only builds the cart image and a ZIP file for uploading release artifacts to GitHub. It would be nice to also add zipped binaries so that people can play the game without needing a PICO-8 image.

• Windows
• Mac OS
• Linux
• Raspberry Pi
• HTML5
• itch.io

See also tducasse/pico8-deploy for adding these Makefile steps.