Modified: Be able to set params from environment variables.

# At fishshell

```sh
> set -x FREE_CLIENT_ID XXXXX
> set -x FREEE_SECRET_KEY XXXXX
> set -x FREEE_APPLICATION_TOKEN XXXXX
> irb
> require 'freee'
> Freee::Base.set_env
> Freee::User.me
{"user"=>{"email"=>"hoge@example.com", "display_name"=>nil, "first_name"=>nil, "last_name"=>nil, "first_name_kana"=>nil, "last_name_kana"=>nil}}
```