Allow negative step count and angle for backward rotation

motor.angle(-90) == motor.angle(90, -1)
motor.step(-500) == motor.step(500, -1)

Stepper.py

@@ -37,6 +37,9 @@ class Stepper:
         
     def step(self, count, direction=1):
         """Rotate count steps. direction = -1 means backwards"""
+        if count<0:
+            direction = -1
+            count = -count
         for x in range(count):
             for bit in self.mode[::direction]:
                 self.pin1(bit[0])