Add ImageRenderer

[Keats]

A renderer that takes the HtmlRenderer and outputs a SVG/PNG like https://carbon.now.sh/

Any added dependency (resvg?) should be gated behind a image flag

[Keats]

See https://old.reddit.com/r/rust/comments/1pqntow/giallo_syntax_highlighting_that_matches_vscode/nuwm1ax/