provide a way to turn off stepper

I can't find away to release the coils on a stepper in motor_featherwing. Could this be added to the library? Similar to release() in Adafruit_MotorShield. Thanks!

[dhalbert]

We'll try to make it consistent with what we do for #17.

[ladyada]

hiya, its now added, call release() to turn off all the coils