Support .env files via godotenv package

[atc0005]

When implemented, the traditional precedence of .env files should be followed.

• https://github.com/bkeepers/dotenv#what-other-env-files-can-i-use
• https://github.com/joho/godotenv#precendence--conventions

This would modify the load order to look something like this:

1. Default values
2. Existing environment variables
3. gotdotenv import of .env files
4. Configuration file
5. Command-line flags

As of this writing we're using the alexflint/go-arg package for flags and environment variable support. My understanding is that by using the godotenv package to load .env files the alexflint/go-arg package will automatically "see" the environment variables and use them. This should make the existing configuration setup seamless.

[atc0005]

See also #3

[atc0005]

Worth noting:

amacneil/dbmate@73e5dda