Bug fix + PAPI expansion

• Fixed dropped heads not being reloaded when /hp reload is run.
• Added an expansion for PlaceholderAPI, of which is set up internally in the plugin so you don't need to install it as an expansion manually. Placeholders currently include:

• %headsplus_xp%
• %headsplus_completed_challenges_total%
• %headsplus_level%
• %headsplus_hunting_% (where is, well, an entity's name, and can be set to "total" or "player".)
• %headsplus_crafting_%
• %headsplus_selling_%

Any other placeholders that can be added in PlaceholderAPI, let me know!