How to use --d4?
Would it be possible to add a bit more information to the readme about how to use `--d4`? Maybe I'm missing something, but I'm failing to get the argument to show up.

I've tried installing d4tools via conda and `nimble install https://github.com/brentp/d4-nim.git` and then installing the current master branch. And the `--d4` argument is still missing. Do I need to add something to the $PATH or something? I feel like I got it to work a year ago or something, so I'm a bit confused :D

I assume, the `when defined(d4):` means it only adds the argument when d4 is available. Wouldn't it be better not to hide it?