feature request: custom controller collider and vehicle control mode

[skywalkershen]

Hi, thanks again for this awesome library, recently I'm developing a bvh collision based vehicle driving mode, and think it would make the library fit more use cases with these features requested:

• custom controller collider: Currently, a capsule is used for simulating the character controlled, it works well with humanoid avatars, yet if saying the controlled model is a vehicle, it is hard to fit the vehicle's shape to a capsule. And the current approach of bvh.shapecast for getting distance and normal of closest point on geometry to capsule center axis segment, may also apply to custom box which can define the vehicle. Or maybe using closestPointToGeometry for random geometry colliders.
• The current controlling mode is a third person control mode that the direction key pressed makes the character facing and moving towards the direction. This can hardly fit into driving mode, in which rather than pressing keys and direct move towards that way, it is more like up and down for accelerating/decelerating towards current direction, left and right for rotating the current direction. It would be nice to have a mode switch to toggle between current control mode and the rotate/acc/dec mode.

I think with the features it will be easier to build different control modes with this library and make it fit into more senarios.

Thanks