0.1.0b Added Difficulty Modes

Added new difficulty modes that allows players to customize their experience:

Casual
- Less zombies
- 1 special enemy type
- Ghost is not active

Standard
- 1 special enemy type
- Ghost is active

Hardcore
- Multiple special enemy types
- Ghost is active

Nightmare
- Multiple special enemy types
- Special enemies can spawn anywhere
- Ghost is active

Additional updates:

• Ghost speed is slower and chase distance is also reduced, to make the game more forgiving.
• Final results screen will now display which difficulty the game was completed on.
• Fixed an invincible spot in one of the closets where enemies couldn't attack player.
• Highlighted the item box to be more noticable to new players.