Add DateFormat to DataType::Datetime

[econaxis]

For my use case, I need to recover the original string as Excel displays it. To do this, I need to know the date format (e.g. "Jan 12 2003" vs "01-12-03").

This PR adds a format value to DataType::Datetime and functions to display the date given a date float and a format specifier.

I will add tests for the new stuff after you give the go-ahead to add this feature to the crate.