stats: when --infer-dates is enabled, also show inferred date format

[jqnatividad]

Currently, with --infer-dates, stats can attempt to parse a column as a date, using 19 date formats
https://docs.rs/qsv-dateparser/latest/qsv_dateparser/#accepted-date-formats

when doing so, add a date_format column with the inferred strftime format string.

This is required by Datapusher+ so users have the option of displaying dates using the inferred date format instead of always showing dates as RFC3339 format - which is perfect for storage in the database, but may not be the preferred way for folks to view the date.