Add LineSpace() for \addlinespace command

[Kiichi]

The \addlinespace[<wd>] command in the booktabs package is quite useful like \toprule and \cmidrule. I can see how introducing the command as LineSpace([wd]) to this package would make it even more appealing.

Like that:

latex_tabular("/tmp/table.tex",
              Tabular("lcl"),
              [Rule(:top),
               [L"\alpha", L"\beta", "sum"],
               Rule(:mid),
               [1, 2, 3],
               LineSpace(),  # convert to "\addlinespace"
               [4, 5, 6],
               Rule(:bottom)])