Would you like add the option to define a icon material with the key of a specify player-skull?

e.g.

{Skull: "eyJ0ZXh0dXJlcyI6eyJTS0lOIjp7InVybCI6Imh0dHA6Ly90ZXh0dXJlcy5taW5lY3JhZnQubmV0L3RleHR1cmUvMjFhNzNkODM2NmNhZWVhNmFlZTY1NmFkZWNkMWUzYmQ1ZjhmN2JlNTcxYTNlMjFjN2FjYjc4ZDQ3NWNiIn19fQ=="}

Added: 42cc919

@Elxafil You can use any NBT data value, including custom_model_data.

I would expand this to allow all NBT values on icons. Something like:
ICON: material {... nbt data ...}

Will you make sure to add in the 1.14 newly added custom_model_data NBT?