Working on knockback (Movable)

This is hard because of the way I have the Movable mixin setup.
But basically you need to add two modes of movement:
normal movement is instantaneous with damping = 0 and
self.v = self.max_v or -self.max_v on left and right movement;
impulsed movement is normal physics movement with damping > 0
and self.v += speed*dt; when an impulse is received switch to
impulsed movement and then when self.v reaches self.max_v or
-self.max_v switch back to normal instantaneous movement...
Too tired to finish this today...;;