Write a configuration library

[msimerson]

that's easily edited by humans and has support for environment settings (production, development, test).

I considered ini but experience with haraka-config left me wanting something more expressive. I looked at TOML but the JS libraries lack maturity. I would consider TOML in the future.

I decided on YAML as the config file language. It's reasonably easy for humans to edit, it's very easy to parse, it works everywhere, and it has mature implementations.

I'm putting config files in ./conf.d organized by where they're used. Currently there's a config for the http and database services.