Fixes the extrude operator to be able to extrude the middle of a curve in a more intuitive way. The extrude operator will still extend the end, but also allows for a curve to be extended from the middle of a curve.
Fixed the crashing issue with a single vertex. It would appear that C is not short circuiting Boolean expressions (or executing them in reverse) that are part of a return statement. I also fixed a minor issue where a single vertex in a cyclical curve couldn't be extruded.