Add support for unique effects and perks, per-gun. Perks are essentially optional special effects of guns that are activated when their bullets hit targets.

Examples

• Speed boost
• HP regen
• "Incendiary" bullets that sets players on fire; player continues to take damage for the next x seconds after being hit by the bullet(s).

Implementation details

• Perks should be registered by using gunslinger.register_perk.
• Effects / Properties of perk should be customizable.
• A gun can register at most one perk, either directly or in its type def.
• During registration, perks should be registered by passing a table containing:
  • Name of perk.
  • Optional properties of perk.