--- chomp([rs])
--- chomp!([rs])

    ʸ rs ǻꤹԶڤޤ
    rsΥǥեͤѿ$/ͤǤ

    rs  nil ꤷ硢Υ᥽åɤϲ⤷ޤ
    rsʸ(ѥ饰ե⡼)ξ硢Ϣ³ԤϤ
    ٤Ƽޤ

    chomp ϲԤʸ֤ޤ
    chomp!  self ѹ֤ޤԤ
    ʤä nil ֤ޤ

    ruby 1.7 feature:
    rs  "\n"(ǥե)ΤȤƥˤ餺 "\r", "\r\n",
    "\n" ΤǤԶڤʸȤߤʤƼޤ

        p "foo\r".chomp    # => "foo"
        p "foo\r\n".chomp  # => "foo"
        p "foo\n".chomp    # => "foo"
        p "foo\n\r".chomp  # => "foo\n"

