Update README
Implement command `all', `help'
Implement command `use`
Implement login process on Baidu service
Replece `console.log` with `debug`
Write back to config file when login successfully
Bug fix, user can reuse the access token previously stored
Bug fix, store current service in a upper level in config file
Refactor function 'debug'
Bug fix, prompt message when use a unsupported service