Add linear mode option

[jonathancrabtree]

Add an option to render Circleator figures as linear plots, without any change in configuration file or input data. Some of the necessary changes:

• A new command-line option (--uncircle or --nocircle or --flat or --linear)
• Current radius would be re-interpreted as figure height
• Circumference would become the figure width
• --pad could be broken into pad_left, pad_right, pad_top, pad_bottom
• no more support for drawing features across the coordinate origin (break them in two?)
• coord_to_circle() method would need to depend on the mode
• all the drawing code would need to check for flat/circle mode
• for text labels:
  • spoke/radial = perpendicular
  • curved = flat/parallel
  • flat/regular = flat/parallel
• all regression tests would need flat versions
• update documentation