ENH: Throttle PID Attenuation

Addresses #1160 based on [baseflight code](https://code.google.com/p/afrodevices/source/browse/trunk/baseflight/src/mw.c#100). Please review / comment, I'm not sure if this is the most optimal way to implement this.
I don't think these variables needing floating point accuracy. It is probably better to store them as uint8 (since we don't need negatives either) and treat them as percentages like we do with lots of other variables.