Support alternative workplane centre options

The default centre option is Centre of Mass (which makes a lot less sense to me than Centre of Bounding Box, but that's by-the-by and at least the bounding box centre is also an option). Often though, I have a face of a workpiece and I want to start drawing at one of the corners of that face.

At the moment, I do this by calculating the dimensions of the face (as I tend to do everything in the model by calculation so fundamental dimensions can be tweaked) and adding a `.center` call or a `.moveTo` call to get to a given corner. This is a lot of boilerplate and it would be nice if there were options to `.workplane` that made the starting/centre point any of the 8 bounding box extremes (North, North-East, East etc).