initialize user with access token

```
import GitHub from 'github-api';

// basic auth
const gh = new GitHub({
username: 'FOO',
password: 'NotFoo'
});
```

how to initialize the user with an access token instead?

No file, test, or entry point is named in the issue. Start by locating the GitHub client authentication initialization and checking whether access-token credentials are already supported; clarify whether the expected result is documentation or a code change, then verify the chosen initialization path works.

索引モデルが issue の本文から書いたものです。