Must The Warrior received bonus damage when he has been attacked by enemies from 'backward'?

[dandgerson]

Hello, WarriorJS!

When the Warrior have being attacked from 'backward' he has recieved normal damage. But I think, that it is logical true, when the warrior have receive damage from 'backward', he must be more vulnerable. But it's about of the balance of the game, and so I can't offer right pull request with my novice programming skills, but i with much pleasure learn, how it's can be perform.

If you agree with this one, and if you'll having commit with this feature, please inform me.