Add Txtble subclasses with default values configured for various common table styles

[jwodder]

• Settings for a reStructuredText "simple table":

  RST_BORDER = BorderStyle(
      hline    = "=",
      vline    = "  ",
      ulcorner = "  ",
      urcorner = "  ",
      llcorner = "  ",
      lrcorner = "  ",
      vrtee    = "  ",
      vltee    = "  ",
      dhtee    = "  ",
      uhtee    = "  ",
      plus     = "  ",
  )
  
  border_style = RST_BORDER
  left_border  = False
  right_border = False

  Note that this does not support indicating column spans with hyphen rules. It also can result in multiline or blank cells in the first column, which rST will misinterpret.

• Settings for a reStructuredText "grid table":

  padding = 1  # Optional
  row_border = True
  header_border = ASCII_EQ_BORDERS

• Settings for a GitHub-Flavored Markdown table, sans alignment-indicating colons:

  GFM_LEFT_BORDER_STYLE = BorderStyle(
      hline = None,
      vline = "| ",
      ulcorner = "| ",
      urcorner = "| ",
      llcorner = "| ",
      lrcorner = "| ",
      vrtee = "| ",
      vltee = "| ",
      dhtee = "| ",
      uhtee = "| ",
      plus = "| ",
  )
  
  GFM_RIGHT_BORDER_STYLE = BorderStyle(
      hline = None,
      vline = " |",
      ulcorner = " |",
      urcorner = " |",
      llcorner = " |",
      lrcorner = " |",
      vrtee = " |",
      vltee = " |",
      dhtee = " |",
      uhtee = " |",
      plus = " |",
  )
  
  GFM_COLUMN_BORDER_STYLE = BorderStyle(
      hline = None,
      vline = " | ",
      ulcorner = " | ",
      urcorner = " | ",
      llcorner = " | ",
      lrcorner = " | ",
      vrtee = " | ",
      vltee = " | ",
      dhtee = " | ",
      uhtee = " | ",
      plus = " | ",
  )
  
  GFM_HEADER_BORDER_STYLE = BorderStyle(
      hline = "-",
      vline = None,
      ulcorner = None,
      urcorner = None,
      llcorner = None,
      lrcorner = None,
      vrtee = "| ",
      vltee = " |",
      dhtee = None,
      uhtee = None,
      plus = " | ",
  )
  
  top_border = False
  bottom_border = False
  left_border = GFM_LEFT_BORDER_STYLE
  right_border = GFM_RIGHT_BORDER_STYLE
  column_border = GFM_COLUMN_BORDER_STYLE
  header_border = GFM_HEADER_BORDER_STYLE

• Settings for a Pandoc Markdown table (requires colsep from Add a VRule class #4 or Add easier ways to configure the column separator #22):

  border = False
  bottom_border = True
  colsep = "  "

• Cf. tabulate's tablefmt