Allow --file-watch to apply to custom files or add --file-watch-extra-files

Motivation: Reload on config file change when config file is in .gitignore

I have a config file called settings.env so I envision:

```
stack build --file-watch --file-watch-extra-files="settings.env,settings2.env"
```

I don't really care how it's implemented though and the above looks ugly TBH. I feel like others frequently encounter the pattern of "settings file in VCS ignore but I'd like to recompile when it changes".