Yeah, the title says it all i think... Gives zombies a chance to spawn with beards. I did this in like half an hour, and it's probably smarter to use an OptiFine resource pack anyways, but... Here it is!

Commands:
/scoreboard players set amount BZSpawnRarity <number>
This command changes the rarity of the zombies with beards spawning. The higher the number, the rarer; set it to 1 and all zombies will spawn with beards.

Also, due to how this pack works, zombies can no longer spawn with helmets on. Also they won't burn in daylight, because they've got custom playerheads on their heads.