Added the ability to put items that the player has permission to wear into the helmet slot manually without using the command (this feature does not work in creative mode).
Changed the default messages in messages.yml for when a player doesn't have permission to wear a hat, and for when the hat's stack size is too large
Notes:
The permissions for putting the hat in the slot manually are the same as the permissions for a command (if a player has Hat.STONE, they can use the command while holding stone, and put stone in their helmet slot manually).
Items that could already be worn as hats are not affected by the change (you won't need to give players permission to wear an iron helmet, for example). The way I implemented this was a little weird, so please let me know in a DM if you notice any abnormal behavior.