I think my preference would be this to take a second, optional argument object which can determine whether the behavior is to fill-in or override.

```js
// Precedence:
//   system environment
//   config/production.env
//   config/default.env
habitat.load([
  "config/production.env",
  "config/default.env"
], { override: false });
```

```js
// Precedence:
//   config/production.env
//   config/default.env
//   system environment
habitat.load([
  "config/default.env"
  "config/production.env",
]);
```

Depending on whether the default for `override` is `true` or `false` would determine whether we have to bump the major version.