Use environment instead of config

Currently, there are two ways to set up the configuration, because each tool is written by the various developers.
One is using `config` and the other is using environments.
Some developers, including me, prefer `config` to environments because the environments vanish when the session closes.
And others prefer environments because it corrupts the git history and it's still hard to track because it depends on environments to choose the configuration file.

But now, we can keep the environment because we use tmuxinator, so I think it's better to unify styles by using environment variables.