chore: support request over proxy

[tasshi-me]

Why

Because cli-kintone v0 unofficially supports requests over proxy by setting the HTTPS_PROXY environment variable.
https://developer.cybozu.io/hc/ja/community/posts/115019277543

What

When the HTTPS_PROXY environment variable is set, cli-kintone sends requests over proxy

• support proxy
  • --proxy cli option
  • HTTPS_PROXY environment variable
  • https_proxy environment variable
• fix and add tests
• update README.md

How to test

yarn install
yarn build

# configure by cli option
node ./cli.js record export --app $APP_ID --proxy <protocol://server:port>

# configure by HTTPS_PROXY environment varibale
HTTPS_PROXY=<protocol://server:port> node ./cli.js record export --app $APP_ID

# configure by https_proxy environment varibale
https_proxy=<protocol://server:port> node ./cli.js record export --app $APP_ID

Checklist

• Read CONTRIBUTING.md
• Updated documentation if it is required.
• Added tests if it is required.
• Passed yarn lint and yarn test on the root directory.