Add aim and PV direction vectors, to support skeletons that have different joint orientations.

[zewt]

When I first tried this plugin I had some trouble figuring out why it wasn't working on some skeletons. It turned out to be something simple: it assumes a particular joint orientation. flipOrientation will fix one particular case, but it wouldn't work in all cases.

I fixed this by adding vectors to specify the aim direction (the local vector from each joint to its child) and pole vector direction (the local vector from the elbow to the pole vector). The default values are (1,0,0) and (0,1,0), which should give the current behavior. flipOrientation is the same as (-1,0,0) and (0,-1,0). These vectors don't have to be axis-aligned.