Allow specifying the config file via an environment variable

[dcastro]

Clarification and motivation

In #19, we're adding a --config option to allow the user to specify a different path for the config file.

We should also allow the user to do this via an environment variable, e.g. COFFER_CONFIG.

When this is done, we should be able to do this:

1. Replace all the tests/golden/**-cmd/config.toml files with a single tests/golden/default-config.toml
2. Set the COFFER_CONFIG env var in the setup function in tests/golden/helpers.bash to make all tests use tests/golden/default-config.toml by default

Notes:

• The CLI option should have precedence over the env var.

This issue is blocked by #21 and #41

Acceptance criteria

• The config file can be specified via an env var
• The CLI option has precedence over the env var
• All the config files in tests/golden have been replaced by a single config file.