Implement scanning custom GitHub Enterprise Server instance

[seqre]

Unfortunately, I do not have access to any GitHub Enterprise Server instance, so I couldn't test it out. It's a relatively simple change, so it should work.

I'm open to any suggestions for improvement.

[bradlarsen]

I think this option can be renamed to be more general? And also more cohesive with the existing options.

It would be nice if there were additional tests to exercise this, but I also don't have easy access to a GHES instance for testing.

[seqre]

Access to the GHES instance for tests is problematic, so I have no idea how to test that functionality. The only tests I could write are to confirm the successful deserialization of OrganizationShort and the CLI returning an error when scanning is requested and a custom GitHub API URL is not provided.

[bradlarsen]

@seqre I made a commit with some additional test cases, a couple small tweaks, and a re-sync with the main branch. I couldn't figure out how to add those commits to this PR directly myself (sometimes that is possible?? maybe to do with "allow edits from maintainers" when creating the PR?).

Instead, I created a new PR with your changes here plus that: #130. You are still credited as a co-author on that in a way that GitHub understands: 855087a

Thank you for the contribution!