replace env files with yaml configs

[jfhbrook]

While working on #1, I realized that there are a lot of issues with using env files:

1. They override externally set env vars
2. They don't respect variables, nor do they expand tilde
3. While munging the env for global config is fine, doing so for app config is less fine

One fix for this is to use godotenv.Read instead of godotenv.Override. But if I already have a go implementation, it seems like a better idea to use viper to load the config - no?