configurable "interesting keys" for monitor command

[jboynyc]

I mostly care about the number of jobs that are currently delayed. It would be nice if the "interesting keys" were configurable (short of editing the function and recompiling).

Thanks for providing this! Interesting to read a Rust project like this to learn about some of the language features.

[schickling]

That's a good idea! Please feel free to contribute this feature as a PR. I'd love to merge this.