I use this template Xcode-Plugin-Template to easily make Xcode 7 plugin. It is used in

• https://github.com/onmyway133/Xcodeway
• https://github.com/onmyway133/Xmas
• https://github.com/onmyway133/XcodeColorSense

For Xcode 8, the evil way is to unsign Xcode https://github.com/inket/update_xcode_plugins

Reference

• How to create Xcode plugin
• Xmas