Hardcore g'rules

Adds some commands and 2 random achievments.

New gamerules:

/gamerule allPlayersDie true

All players die when 1 player dies.

/gamerule reset true

On death, all items are wiped and you get tped to a random set of coordinates between -1000000/1000000 x and z.

/gamerule noKnockback

actually removes damage cooldown and knockback. Fall into lava and you will get oblitterated