feat: Support for auto-sizing specific columns using None

[DenverCoder1]

None can now be used to specify that specific column(s) should be automatically sized.

Example:

table2ascii(
        header=["#", "G", "H", "R", "S"],
        body=[["1", "30", "40", "35", "30"], ["2", "30", "40", "35", "30"]],
        footer=["TOTL", "130", "140", "135", "130"],
        column_widths=[7, None, 5, 5, None],
)

Additionally, improvements to the docs.