Add an HRule class

[jwodder]

Add an "HRule" class that can be passed in place of a data row to indicate that a horizontal rule should be inserted there in the resulting table.

• HRule constructor arguments (all keyword-only):

  • fill: Optional[str] = None
  • joint: Optional[str] = None
  • left_cap: Optional[str] = None
  • right_cap: Optional[str] = None
  • border_style: Optional[BorderStyle] = None
    • When specified, border_style is used to fill in the default values for the other parameters. When border_style is None:
      • If fill is None, border_style inherits from the Txtble's row_border (or from border_style if that is not a BorderStyle)
      • If fill is not None, ???

• Give HRule options for (a) not applying the rule's style to the table border intersections at the ends and (b) not applying the rule's style at column intersections?

  • Somehow give header_border and row_border analogous options?
  • Allow header_border and row_border to be set to HRules?

• Give HRule options for controlling what happens when adjacent to a top_border, header_border, row_border, or bottom_border (possibilities: *_border takes precedence, HRule takes precedence, draw both)

Some basic test cases for this feature can be found on the feature/hrule branch.