Potions will proved an effect that lasts until the player moves to the
next level. So far there's a frost potion which adds armor and makes one
immune to tile effects (fire, gas) and a bloodlust potion which is
essentially quad damage.

This might make the player slightly OP. But this late. I don't think
anyone will complain.

A maximum of one potion will drop per level.

Future potions might be:

- Levitation: Avoid pits (white)
- Speed: Rogue move speed (yellow)
- Barkskin: Armor (brown)
- Extra life: Avoid one death (green)