Update 0.13

After a long while, here's finally an update. You can now select guns between levels, and there's some extra stuff too.

I'm going to add more weapons and wielded items (like shield or grappling hook) before redesigning the upgrades system, so they'll be all unlocked for now.

• Added selectable wielded items.
  
• Added plasma gun (powerful, but overheats quickly).
  
• Higher enemy health in later levels.
  
• New explosions and smoke.
  
• Screen shake (slider in options).
  
• New HUD.
  
• Miscellaneous technical updates.