Copter: Guided mode that supports position and velocity

There are two ways we could interpret a SET_POSITION_TARGET with a position and velocity:
1. The position target is stationary and the copter is to fly through it at the provided velocity. Useful for path following.
2. The position target is moving at the provided velocity and the copter is to track it. More useful for e.g. followme.

This pull request interprets it as 2. Upon receiving the command, the copter sets the target position to the supplied position target, moves it at the supplied velocity and tries to track it. This is perfect for e.g. followme.