Feature request: sort columns by date and time.

[derekmahar]

Please add a date and time data type to the existing csvtk sort command in order to sort columns by date and time. This would require that the user specify not only the columns to sort, but the date and time string format of each of these columns so that csvtk can parse the date and time strings in the corresponding columns, convert them to an internal time representation for comparison between rows, and order the rows in ascending or descending order by time.