Make stack kill executed process when running `stack build --file-watch --exec "foo"`

When I am programming a long running application (like a web api), I'd like to have a feature where the program that has been executed after building is killed when a source code change is found.

For example, I am building an api using Servant and I would like to run it like this:

```
$ stack build --file-watch --exec "stack exec -- my-api"
```

Everytime I change one of my source files, it would be nice if `stack` automatically killed and restarted `my-api`.

Currently `stack` just waits until `my-api` is finished running (which never happens).