Line API: Take center as argument

[nikochiko]

It will be consistent with the rest of shape functions that we provide.

It will also be more convenient to use with transformations, once the line is created. Scaling a line that is not centered will not be symmetrical around its center.

[FayCarsons]

Can you please expand on this? I'm not sure if you're suggesting renaming the first optional point to center or that there should be an additional point that acts as a center point.