Using attributes for skill level

You can use attributes on items to apply buffs to skill levels.

This works by hijacking the attribute setting its name to the desired skill, example as follows:

/give @p leather_chestplate{AttributeModifiers:[{AttributeName:"skillz:item_bonus",Name:"skillz:melee",Amount:2,Operation:0,UUID:[I;1170423212,2071743757,-1220810514,1287223732],Slot:"chest"}]} 1

That is an chestplate that gives an additional 2 skill points to the melee skill when worn.