Add --oauth-token option to allow oauth token authentication

In my case, I don't want to write plain text password at Jenkins's configuration textarea.
GitHub's Repo Status API (and octokit) allows oauth token authentication, so I added `--oauth-token` option.

In addition, oauth token can restrict permission by scopes.
If `--oauth-token` option is accepted, the users of github-commit-status-updater can choose more secure authentication mechanism using `repo:status` scope.

Usage:

```
$ ./bin/github-commit-status-updater success -r kyanny/test -s 17e993377f82a6dd02fe92111d1b7d68de26a333 -u kyanny --oauth-token=(masked)
status success
```

Example pull request: https://github.com/kyanny/test/pull/2