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.
this kind of boolean handling of a floating point variable is bad practice. you should probably do something like (tpa_attenuation[i] > 0) since the code doesn't make sense with a negative.