table: support sorting by any of the columns

[jedib0t]

Support sorting tables by any of the columns. Example:

    tw := table.NewWriter()
    tw.AppendHeader(Row{"#", "First Name", "Last Name", "Salary"})
    tw.AppendRows([]Row{
        {1, "Arya", "Stark", 3000},
        {11, "Sansa", "Stark", 6000},
        {20, "Jon", "Snow", 2000, "You know nothing, Jon Snow!"},
        {300, "Tyrion", "Lannister", 5000},
    }
    tw.SortBy([]SortBy{
        {Name: "Last Name", Mode: Asc},
        {Name: "First Name", Mode: Asc},
        {Number: 4, Mode: DscNumeric}, // corresponds to the "Salary" column
    })
    fmt.Println(tw.Render())

[coveralls]

Coverage decreased (-0.1%) to 99.866% when pulling cb9c356 on table-sort into 844e6f1 on master.