Project idea: port markdownlint to Rust

People are always looking for simple projects to learn Rust with, so here's one for anyone who's currently looking. Port markdownlint to Rust. Markdownlint is ~3.3k lines of JavaScript (including the lint implementation themselves!), so I reckon it's pretty doable.

Motivation:

• Markdownlint is Taffy's slowest CI job (taking a whole 2 minutes - yes we're spoiled with fast CI). It would be nice to speed that up.
• It's also used by some high-profile projects that might also like a speed boost

Recommended crates:

• https://github.com/clap-rs/clap for argument parsing
• https://github.com/BurntSushi/walkdir for discovering markdown files
• https://github.com/wooorm/markdown-rs for parsing markdown
• Either https://github.com/chyh1990/yaml-rust or https://github.com/dtolnay/serde-yaml for parsing the YAML config file that markdownlint uses