Allow forcing ordered lists to start with alphabetic instead of roman numerals

[jakov]

When continuing a list with "i.", pandoc supposes that you want to use a roman numeral list. Until now there is no way to force alphabetic treatment. The same applies the other way around: "m." is unchangeably interpreted as alphabetic.

Resulting from a short discussion about it (https://groups.google.com/forum/m/?fromgroups#!topic/pandoc-discuss/ES4fUR19MDg) i suggest the following syntaxes:

A roman numeral should be forced to alphabetic (and the other way around) by putting three spaces instead of only one (or maybe ":") after the dot: "i.^^^" (or "i:^") would get alphabetic instead of roman; "m.^^^" ("m:^") would gte roman instead of alphabetic.

Also "v." and "x." should default to roman numerals.