Clumsy attempt to add subscript support

[mdaeron]

This PR adds support for subscripts, using a tilde notation equivalent to the superscript ^:

• N~2 --> N₂
• H~(2)O) --> H₂O

For now superscripts and subscripts are activated together by the --superscript option.

The problem is that --superscript now disables --strikethrough, which also uses tilde (e.g., ~~foo~~ --> foo).

I'm worthless with C code, so I couldn't find a way to fix --strikethrough while preserving subscripts. Any help would be much appreciated, even if it's not merged here (I'd be fine with building my own tweaked version of MacDown for personal use).

[uranusjr]

Could you explain a bit the rationale of choosing ~ for this? It feels slightly foreign to me; LaTeX, which also uses ^ for superscript, uses _ instead. I am not sure what other Markdown renderers tend to use (IIRC CommonMark does not support either).