Add MirrorURL field to Repository

The `mirror_url` field on repository objects is the clone URL of the mirror's origin repository, or `null` if the repository is not a mirror. To see an example JSON response for a mirror repository, run `curl https://api.github.com/repos/apache/avro`.

The `mirror_url` field is currently immutable (according to [this post](https://issues.apache.org/jira/browse/INFRA-6130)) and determined upon repository creation. (I mention this only because I don't think it'd be possible to set the `mirror_url` field to `null` with the current struct field-pointer scheme. As long as it's immutable, this is not an issue.)

BTW, I was unable to create a mirror repository with my user account (POSTing a repository with `mirror_url` set just created a normal repository with `mirror_url: null`), so it's possible that it requires special privileges. Can anyone confirm this?