Peculiar Mobs

Adds few mobs to make the game more challenging:

1. Fast zombie: a zombie which moves as fast as player;
2. Reachy Skeleton: skeleton whose attack range is twice bigger;
3. Web Creeper: creeper which makes web explosion;
4. Reachy Creeper: creeper which ignites from a farther distance.

There are no spawn eggs of them, use command if necessary.