Give missiles an attribute that determines how susceptible they are to countermeasures

[IrateRedKite]

I would like to be able to have different missiles have varied resiliency against countermeasures, instead of this only being based on the effectiveness % of the CM itself.

This could be set up as a modifier against the % chance to work of CMs. For example a missile with a modifier of 2 against a CM with a diversion_pctg of 10 would have a 20% chance of being directed. Conversely, a missile here with a modifier of 0.5 would have a 5% chance of being diverted.