Class member callback

Actually I'm just asking for a favour, because it's not really an issue. 

Could a class member callback also be implemented? In the way it's done with the Software Timers: https://docs.particle.io/reference/firmware/photon/#software-timers

If I knew how to do it, I'd do it myself but I think this is a bit outside my scope. 

It would be a nice feature. 
