Feature request: Superscript/subscript support in text strings

Useful for plots that include chemical names, variables, etc. (example: R²).

A possible way to implement this would be to support a subset of Markdown or HTML syntax (for subscript and superscript, and possibly but less importantly bold and italics). There already exist Go parsers for both, so the tricky part would be implementing the text sizing and offsetting in all of the vg backends.

A more powerful alternative would be to support Latex syntax, but I feel like the difficulty to usefulness ratio would be too high.