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.
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