Add an option to use Roman numerals in Latin locale

[tukusejssirs]

I think romcal should have an option to use Roman numerals in Latin for years. For days of week, Roman numerals should be always used.

For other locales, this option would have no effect, unless they also used Roman numerals (ATM, I can’t think of any locale other than Latin that would use Roman numerals for years).

See also this comment.