Support for GitHub Enterprise

[rpau]

I could not find a way to setup the GitHub Enterprise API URL to load the corresponding repos/orgs instead of the public GitHub.

Is it possible? because as a user, I'd like to test source{d} with the repos of my team.

[carlosms]

No, this is not yet on the backlog roadmap.

Hopefully this can be added easily with a new option and https://godoc.org/github.com/google/go-github/github#NewEnterpriseClient, but we would also need to figure out how to test it properly.

[rpau]

Regarding the tests, I hope that this comment helps:

GHEs are old versions of the public GH, and the APIs are retro-compatible. The only bit that changes is the base URL: https://developer.github.com/enterprise/2.18/v3/#schema

[se7entyse7en]

@rpau shall we close this in favor of a proper epic/story?