Add support for global .env file

The global .env file is designed to sit in your home directory and provide defaults for any env vars that you might want.  Each key is overridable within a project specific .env file.

For instance, you may set your local Postgres server credentials in your global .env and use them inside your database.yml, but want to have project specific settings for connecting to a different postgres service.

**NOTE: DO NOT MERGE THIS YET!**

There is a pending test in cli_spec which I need help with. I'm unclear on how to create a test that represents what is happening.  In true back to front fashion though, I know the code is working.  Help welcome.