Add CLI support

Adjusted simple CLI code from @sindresorhus, see for example [sindresorhus/latest-version](https://github.com/sindresorhus/latest-version).

Just returns raw JSON to the CLI:
```sh
$ google-spreadsheets-key-parser https://docs.google.com/spreadsheets/d/1jHY4wO4b0kuX4rVnJgZGwfQUzoxado52k1hzxdUY-AM/edit?pli=1#gid=0
{ key: '1jHY4wO4b0kuX4rVnJgZGwfQUzoxado52k1hzxdUY-AM',
  isNewSheets: true }
```