Limit yourself

New version with some breaking changes:

• New command-line options:
  • -t/--total-lines for specifying maximum number of lines to be generated
  • -l/--line-limit for limiting the number of lines that are generated in one command invocation,
• Library additions/changes:
  • Added functionality to implement first two points on this list.
  • Added an option to set a callback when calling plot_spiral, called once every time a new line is generated.
• General tidying up.