Subscript support

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.

Start by reviewing how MacDown currently parses and renders Markdown, then determine which subscript syntax should be supported from the alternatives proposed in the issue. Done means an agreed syntax works reliably for scientific text in the macOS editor and its behavior is covered by verification appropriate to the existing project.

Written by the indexing model from the issue text.