Add a parameter for custom date formatting strings

[EliteLoser]

Add a parameter for custom date formatting strings, as supported by the ToString() method called on DateTime object types. I should have done this in the first place and just defaulted to ISO8601 seen in retrospect.

Now I'm stuck with the -DateTimeAsISO8601 parameter, so I will add a new one, maybe with the name -DateTimeFormatString - and make it so you can't specify both of these at the same time, either with parameter sets or just conditional if/else statements, likely the latter.