Subscript support

[mdaeron]

Markdown has been gaining traction for scientific texts over the past few years. Subscripts are quite frequent in many fields (e.g., chemistry and physics, obviously). It would be nice to have at least some version of subscripts. FWIW I personally prefer H~2~O to H~(2)O and ^14^C to ^(14)C for reasons of consistency with other delimiters, but anything functional would be great, frankly.

[ajcullum]

Seconding this request. It's interesting that superscript support via ^ already exists, but not subscripting.

[pink-ink]

I also would like subscript to work.