add a rule for ROMAN numbers

[kosloot]

it 'might' be a good idea to add a rule for ROMAN numbers. A rule along these line: ROMAN-NUMBER=(^M{0,4}(CM|CD|D?C{0,3})(XC|XL|L?X{0,3})(IX|IV|V?I{0,3})$) If we add this, do we also want lower-cased variants? We have some use cases for them, like in the DPO files.