load tests via a config file

I have a local branch working on this, and realized since I had not mentioned it in the repo that I should probably let folks know I was working on it. So this is a tracking branch for adding an api like this:

```
$ echo '{"overrides":{"express": "5.0.0"}}' > config.json
$ expf --config config.json
```

Basically moving a bunch of things that also have flags to be passed as a json config enabling us to express more complex needs without having really rough cli flags. I hope to be able to open a PR for it sometime soon but just in case wanted to track it here in case other folks were thinking of a similar thing.

Add support for env variables.