`stack ghcid`

I switched from using `stack test --file-watch` to `ghcid`, using a script I hacked together:

```sh
ghcid -c="stack ghci test/QuickCheck.hs" -T="main" $@
```
This gives me instant rebuild and full test suite on every save (to my source *and* test suite). It's so fast it's scary. And it politely clears the screen behind itself between runs.

However, this script doesn't support multiple test suites, and it's not clear how to without duplicating information already in `package.yaml`.

This badly needs proper stack integration. I assume stack intends to support this use case in principle, since it has `--file-watch`, and `ghci` is vastly better.