This adds support for using the GH_CONFIG_DIR environment variable to determine
where the config files are written, this is useful for cases where the homedir
is not writable.
After a person copies `hosts.yml` to a headless system, they will still
be forced to go through re-authentication flow unless they copy or
initialize a `config.yml` as well. This fixes respecting authentication
info from `hosts.yml` even if `config.yml` is missing.
Read from and write to the `hosts.yml` file every time `config.yml` is
accessed. Everything that before went under the `hosts:` map now belongs
to `hosts.yml`.
