Thruster at an offset

[namheegordonkim]

Currently, the external assistive forces are applied at the center of mass, but for many shapes, it makes more sense to attach thrusters at the vertices/boundaries of the bodies instead. Supporting offsets would also allow multiple thrusters to be attached to the same body. I couldn't find anywhere if this is supported yet. If not, I'd be happy to make one and work towards a PR.

[erikfrey]

This seems like a very reasonable request. After all, joints have offsets, too. OK, if you open a PR we'll get this in.