Sync teams across multiple GitHub installations

This is a spike to sync teams across GitHub installations. The goal here is to allow a team on GitHub.com to sync to or from a GitHub Enterprise installation.

Things left to do:

- [x] Allow syncing to or from a GitHub Enterprise installation (currently, you can only sync *to* a GitHub Enterprise installation)
- [x] Change `destination` to the existing `target` wording

@mtodd I'm killing off my fork of this repo and working here now. :grinning:
Missing docs on this ENV var or stick with `SOURCE_TOKEN` on this too.