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.
Yes.. I thought it was ugly as well but I saw it elsewhere in the code, so I did the same. I will fix it.