Please add some pre-action features:enhancement

[PudgeKim]

I hope that this kind of feature is added.

1. Register some function to the bonsai bt.

fn check_sleep(..) -> (Status, f64) {
    if monster.is_slept() {
        return (Running, 0.0)
    }
    ... // do next action

2. Select actions to which the function is called.

bt.register(check_sleep, vec![Monster::Attack, Monster::Walk, ...]);

3. Registered function is called before selected action is called in a behavior tree.