message: feat(Step): add transition
diff: &__line {
  position: absolute;
  background-color: @step-line-color;
  + transition: background-color @animation-duration-base;
  }
  &--horizontal {
  &__icon,
  &__title {
  + transition: color @animation-duration-base;
  +
  &--active {
  color: @step-active-color;
  }
  