ArduPlane: Added support for separated elevator servos

Hi Guys,
I thought I'd start my first contribution with something relatively simple. One of my projects involves a larger aircraft with separated elevators that use individual trims and radio inputs. I'm following the recent k_aileron_with_input addition by Tridge to add both an auxiliary servo that follows the main elevator channel, and an elevator with its own radio input similar to k_aileron_with_input.

Please let me know if this is a satisfactory contribution or if there is anything I missed/needs changing. The code change has been compiled successfully, but not been tested as of yet. This can be done before you pull it in if desired.

The changes also include an uncrustification in the RC_Channel library.