More control over formation changes.

Allow to use :average1 .. :average6 and :fast1 .. :fast3 to allow more control over what player moves to a newer position.
The old way a single change in movement would cause a lot of movement and often leave the team vulnerable.

NOTE: This change is backward compatible. i.e.: the old way of specifying formations will still work.
I appreciate you've taken care of backward compatibility. However, I think we don't need to have more complicated code because of that since this gem should not be consumed by any application. People with previous versions of the Coach instances can still play with the according released version.

Does it make sense to you? Could we just leave the new version?