Allow getting a repository by ID

[jamesatha]

Other clients use the undocumented api to get a repository by ID. Can we add that to this client as well?

Examples:
PHP: KnpLabs/php-github-api#579
Python: https://github.com/PyGithub/PyGithub/blob/b64b0d6942bb27095bd035b8c0db10ca35448be3/github/MainClass.py#L191
OctoKit supports it: octokit/rest.js#163 (comment)

And it seems to be sticking around indefinitely: piotrmurach/github#283 (comment)