Allow mirroring of private Github user repos

At present, the mirroring script permits the mirroring of any of a Github organisation's repos - public or private - but only public repos owned by users. This appears to be because the endpoint we use to list repos (/users/:username/repos) only returns public repositories. Given, however, that the personal access code supplied in the config should grant us the full repository permissions of that user, we should be able to use /user/repos as well (or instead) in order to pick up their private repos.