Added menu entry to adjust current step of encoder

Added menu entry to adjust the step size of the encoder. The following values are selectable: 1, 2, 5, 10, 20, 50, 100, 200, 500 mA.
The default value after reset is 20mA as defined in config.h: CURRENT_STEP.

This is intended as a temporary workaround for issues #27 an #43 until a better solution is implemented.