[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.change)* **Changes the existing Skull to the entered value.**

*While NOT looking at an existing skull...*
 >*(essentials.skull.others)* **Gives the sender the Skull of the entered value.**

**Has been compiled & tested locally.**