Keep the camera's initial yaw/pitch, unless some are explicitly provided to FlyCamera

[Gnurfos]

Without this, if you do something like:

let cam_transform = Transform::from_xyz(15.0, 15.0, 15.0).looking_at(Vec3::new(5.0, 5.0, 5.0), Vec3::Y);
let mut cam_bundle = commands.spawn_bundle(PerspectiveCameraBundle {
    transform: cam_transform,
    ..Default::default()
});
cam_bundle.insert(FlyCamera {
  ..Default::default()

});

The transform you set will be lost.
This Pull Request makes the FlyCamera take its initial yaw/pitch from the real camera.

It changes some field names. Let me know if I should do something backward-compatible instead (I suppose the code would be a bit more complicated).