Add support for phase/enable driver interface.

[lcmcninch]

DC motor drivers like the DRV8256E use a phase/enable interface. This interface requires a single PWM "enable" signal and a direction (phase) signal, as opposed to the dual-PWM IN/IN interface used by drivers like the Adafruit DRV8833. From what I can tell, this library doesn't currently support the phase/enable interface, only the IN/IN interface.

The first question is: Am I overlooking something?

If not, I have a fork that enhances the DCMotor class to optionally support this driver interface. I'm not sure how common this type of driver is. Would you be interested in a pull request with these changes?