isn't that an implementation issue? your example doesn't work indeed in the StackEdit link, but

1. Item one

1. Item two

   1. Subitem a

   1. Subitem b

does work, so that let me think that you just need to improve the parser to consider "a b c" as a valid ordered list, and maybe "i ii iii" at the same time?

(I replaced a, b and 2 by 1 to force the automatic numbering)