Support for AdvancedFirmata stepper control protocol

This implements the protocol in AdvancedFirmata (https://github.com/soundanalogous/AdvancedFirmata) that will allow asynchronous control of stepper motors.

Tests are written and everything passes lint and unit tests.

The only thing I am not sure of is the function signatures for stepperConfig and stepperStep, they seem a bit long to me, and if you think it would be better to collapse them into an object that is passed in, I can do that and resubmit

Let me know if you see anything that needs to be changed
Thanks!