Allow .env.local overrides

For context, we're checking `.env` into source control with default development environment settings.

However, like #111 we want to override these on a per-machine basis. Previously we were using `.env.development`, but this has since been deprecated.

I want to propose a new `.env.local` as a standard recommended place to keep machine-specific overrides, with the idea being that it's not checked into source control.

Thoughts?