[Feature] Add /skull command for giving and modifying skulls

*This is my first time using Netbeans so bare with me if the formatting is out.*

**Description:** This PR creates the /skull command, which has two main uses - Giving and Modifying (existing) Skulls.

**Command Structure:**

⋅⋅⋅ /skull *(essentials.skull.own)* **Gives the sender his/her own Skull.**
⋅⋅⋅⋅⋅⋅/skull <name> 
⋅⋅⋅⋅⋅⋅⋅⋅⋅*While looking at an existing skull...*
⋅⋅⋅⋅⋅⋅ *(essentials.skull.own)* **Gives the sender his/her own Skull.**