The plugin should contain a link to this: Custom Controls — OctoPrint master documentation
as-is, it is hard to understand what this is, and how to use it.
Also: please add an option to execute shell commands/(in my case, for GPIO control)
The bundled custom control manager that is included with the next release of OctoPrint will expose the settings. Still no system command support, but for that you can use system commands or gcode system commands plugin in combination with custom controls.