Add rune drop chance control

[Deadlock39]

Add a control knob that affects rune drop chances.

Minimum setting is vanilla drop rates
Maximum setting is equal drop chance for every rune any time a rune TC is selected.
Use logarithmic scaling instead of linear to give more intuitive results.

Example:
When TC 17 is selected Zod drops at a 0.02% chance.
At them maximum, it will drop at a 3% (1/33) chance
At half of the maximum setting it should drop at a 0.25% chance (0.02/0.25 = 0.25/3) instead of a 1.51% chance

This is done using log scaling.
Instead of:
chance = minChance + (maxChance - minChance) * scaleFactor
Use:
chance = Pow( 2, Log2(minChance) + (Log2(maxChance) - Log2(minChance) ) * scaleFactor )

[Deadlock39]

I've written and minimally tested this code. It looks good. Need to finish documentation and push patches.