Support alternative GitHub instances (GitHub Enterprise Server etc.)

[jwodder]

labelmaker should gain an --api-url command-line option (defaulting to the value of the GITHUB_API_URL envvar, falling back to https://api.github.com) for specifying what GitHub API to interact with.

A preliminary implementation of this was removed in commit 6702d94.

Problems:

• The only repository URLs (whether HTTP URLs specified on the command line or SSH URLs retrieved from a local Git repository's remote) that ghrepo can parse are those that refer to the main GitHub instance.

• Given an API URL, how should the --hostname value to pass to gh auth token be determined?

  • It seems that, if a token is stored under example.com, then gh will return that token for all queries of subdomains under example.com.

As an alternative to --api-url, support could be added for specifying repositories in the form HOST/OWNER/REPO à la gh.