Accept the "HOST/OWNER/REPO" syntax or passing a full URL for both the
`--repo` flag and the GH_REPO environment variable and allow setting
GH_HOST environment variable to override just the hostname for
operations that assume "github.com" by default.
Examples:
$ gh repo clone example.org/owner/repo
$ GH_HOST=example.org gh repo clone repo
$ GH_HOST=example.org gh api user
$ GH_HOST=example.org gh gist create myfile.txt
$ gh issue list -R example.org/owner/repo
$ gh issue list -R https://example.org/owner/repo.git
$ GH_REPO=example.org/owner/repo gh issue list
