Add support for undocumented endpoint to get user by id

[stocks29]

Although undocumented, there is an endpoint to get a github user by id:

https://stackoverflow.com/questions/11976393/get-github-username-by-id/30579888#30579888

https://api.github.com/user/1

It would be nice to add support for this although docs probably need to state that it's undocumented.