markup_regex: recognize only valid roman numerals
For ordered lists the prefix recognized any [a-z]+ string. That caused an issue if a line started with a [a-z]+ string followed by a dot, if that string was not a single letter or a valid roman numeral. That would dedent the line in the editor pane as if the string was a prefix. This changes the regular expression to recognize as prefix only: - one or more digits - or a single lower case letter - or a valid roman numeral in lower case letters Fixes #418.