def [](row_index, column_index = false)
      return row(row_index) unless column_index
      if row_index >= 0 && column_index >= 0
        rows.fetch(row_index) { return nil }[column_index]
      else
        fail TTY::Table::TupleMissing.new(row_index, column_index)
      end
    end
def [](row_index, column_index = false)
      return row(row_index) unless column_index
      if row_index >= 0 && column_index >= 0
        rows.fetch(row_index) { return nil }[column_index]
      else
        fail TTY::Table::TupleMissing.new(row_index, column_index)
      end
    end
Lookup element of the table given a row(i) and column(j)

 @param [Integer] row_index
 @param [Integer] column_index

 @example
   table = TTY::Table.new [['a1','a2'], ['b1','b2']]
   table[0]    # => ['a1','a2']
   table[0,0]  # => 'a1'
   table[-1]   # => ['b1','b2']

 @api public