elixir-formatter is displaying an error on save

With phoenix version 1.4 came the inclusion of the formatter...


However, this is causing the following error...


This line causing the issue is...
```elixir
url: [host: Map.fetch!(System.get_env(), "APP_HOST")],
```
in the config file.

This seems to be happening because the formatter has not got access to env variables that the app would when the application is compiled (assuming you have sourced your .env file of course).

There may be a simple solution to this issue that allows us to both format on save but also keep the safety that the `Map.fetch!` syntax provides (it does not allow you to run the app if you have not set an env file that is needed in the applicaton (VERY VERY HELPFUL for people like me who forget to source envs far too often).

Haven't looking into solution yet. Just wanted to capture this issue for now.