Add CLI sub-commands and support running external commands in env(s)

[rsyring]

# `env-config db-prod api-dev` becomes
$ env-config load db-prod api-dev 

# `env-config --list` becomes
$ env-config list

etc.

Run External Command

# Canonical
$ env-config run --env db-prod --env api-dev pg_dump ...

# Shortcut version for less typing
env-config run --env db-prod,api-dev pg_dump ...

[bchopson]

We could also specify the external command after -- like the 1password cli command op run -- <your_command>