Some more work on mrh:
- A friend did me a nice favor of creating an Iterator for Crawler, a task I failed at after 2 attempts... I don't yet truly get Rust lifetimes
- Based on that work, I turned Crawler itself into an Iterator
- I added YAML and JSON output; they are behind a compile flag, since they bring with them some slow-compiling dependencies, and I don't expect the features would be a common need