Balance hybrid throttle with throttle_factor

WARNING : CODE NOT TESTED - mind to enable STABILIZE mode on your switch
to quickly retrieve control in case of!

Added HYBRID_THROTTLE_FACTOR. Used to define the min and max throttle
from the throttle_cruise in hybrid mode. Should be between 1,1 (smooth)
and 1,5 (very strong). Actually, this range has to be defined. We'll see
if it requires a param.

throttle_min=throttle_cruise/HYBRID_THROTTLE_FACTOR
throttle_max=throttle_cruise*HYBRID_THROTTLE_FACTOR