Add easier ways to configure the column separator

[jwodder]

• Add a colsep: str option for setting the column separator to a constant string (Cf. the option of the same name from Add a VRule class #4)
  • Let it be a sequence of strings in order to support "doubling" of specific vrules
• Add an option for setting the column separator only for non-hrule rows (i.e., the separator between adjacent cells) (Cf. Support configuring "joint" border styles #9)
• Allow column_border, left_border, and right_border to be just a string? (since only one string of BorderStyle gets used anyway)