Add support for #booking_datetime and #value_datetime

[danielpuglisi]

I just stumbled upon a camt.053 statement that had a booking datetime value instead of a booking date.

This PR adds support for Entry#booking_datetime and Entry#value_datetime while also returning nil for Entry#booking_date and Entry#value_date if these attributes are empty. This makes it possible to simply check for nil when otherwise Date.parse(nil) would have thrown an ArgumentError.

For example:

booking_date = entry.date || entry.datetime&.to_date

[tobischo]

Hi @danielpuglisi, thank you for the contribution.

I will take another look hopefully in the next few days, but it seems reasonable